So this card is extremely interesting to me, maybe one of the most interesting cards in this cycle so far. It provides a new effect in mystic, being able to recur your events, but comes with some significant restrictions.
I think first of all I want to discuss the value of the effect here. It can sort of be considered to be a form of card draw, since you are getting access to a new card. But if you have multiple events in your discard pile it can also be card selection, rather than just gaining an extra card you get to choose from several. Then, with the same action as drawing the card you get to play it. So in a sense it can be considered to be compressing an upgraded draw action and a "play an event" action into a single action. Then you add on the resource cost reduction, and it can sort of be considered to be 3 actions worth of effect in one action. However, I think this is actually underselling the power of the card. Because you aren't just getting to draw and play a card but you are also getting extra uses of a given effect. If you are building around this card and playing lots of powerful Spell and Insight events then you may be doubling the number of times you get to use the most powerful cards in your deck. That's much more powerful than just drawing a card. So obviously, this effect has to come with some downsides or it would be busted. The downsides are pretty significant, so I want to break them down one by one.
Firstly, putting a doom on itself to use the ability means one of a few things: Either you are waiting to use this during the midnight hour, you are losing a turn to use it, or you are combining it with something like Moonlight Ritual to handle the doom. Most likely you are probably wanting to do some combination of those things, and think this flexibility with how you handle the doom makes this cost a little less steep than it initially seems. Most scenarios will give you at least 2 free uses of this card during the midnight hour, with many giving you more than that. 2xp, 2 resources and a pre-emptive action to get to recur 2+ events is already fairly reasonable though not amazing (and of course bear in mind that you might not be able to draw in time to use all those midnight hours). Some knowledge of scenarios helps here as there are some scenarios that have significantly more than 2 midnight hours (Depths of Yoth comes to mind) so there may be campaigns that this over-performs on in this regard.
Adding to these considerations around the midnight hour, you can also add the fact that sometimes you just have spare turns on some scenarios, or have events that are worth more than a single turn. This is probably a less common scenario due to some properties of the specific events this can be used with (I'll discuss this in a minute), but it bears mentioning. The Pallid Mask in particular has a very long last agenda, with your time instead being constrained by the specific act 3.
Managing the doom on this card with Moonlight Ritual can also be worthwhile, this lets you go ham on using this card every turn and then clear it off right before the agenda advances. Amusingly, Moonlight Ritual is Insight and spell traited, so you can actually use this to replay your ritual and clear off all the doom including the one you just placed. Worst case, you can also manage the doom by just playing an asset over this too. If you are building around this plan then you're going to want to be running a lot of events that are generically powerful, so that you can expect to fire one off every turn without having to contort your gameplan too much. This is a little easier said than done because of the sort of cards that are Spell or Insight traited, but I'll get to that.
The next restriction is the symbol. This makes the card a little awkward, as Fast cards now take an action, cards with Fight or Evade now provoke attacks of opportunity. The former case isn't the end of the world, making Fast cards slow is sort of off-set by the cost reduction, but you'd still rather be using this on cards that take a full action normally. The latter though makes those cards a lot less appealing, get out of dodge cards like Banish are suddenly a lot less safe, and trying to repeatedly fight using spell events is going to run out your health pool very quickly. This also importantly means that reactive cards like Ward of Protection can't be recurred, which is a shame because those are some of the more powerful cards to play multiple times.
It also removes the event from the game, this basically makes sense for this sort of recursion card and I dont think there's a whole lot else to say about it.
This also only works on Spell and Insight cards, which is quite a long list particularly in Mystic. So this gives us a set of restrictions on the sort of events we want to include to synergize with this card: spell or insight cards that are still functional and worthwhile as a and which are ideally worth losing a turn for. There's also the minor point that in theory you can re-play cards that you've committed to tests rather than ones you've played, so like I suppose if an event has good icons thats worth considering.
I've gone ahead and compiled a list of noteworthy cards by class:
Emergency Aid - This seems potentially very worthwhile, saving yourself or an ally from death is usually worth more than a turn, and this can be used as a throwaway event for a midnight hour usage.
Scene of the Crime - You will suffer AoOs if the enemy is engaged with you, but you can use this when an enemy is engaged with someone else, or an aloof enemy. It is even more restrictive than De Vermis Mysteriis normally would be, because of the bold restriction. However, you're likely already playing this card in a lot of guardian decks so its all upside.
Astral Travel - if you were playing this card anyway you'll probably want it for 1 resource less. There are some scenarios where jumping about the map is saving you a lot more than 1 turn worth of actions, particularly in Carcosa. However, for a lot of scenarios where Astral Travel is good its only actually good once. Escaping the Doom of Eztli for example. There's the corner case where you commit Astral Travel to a test to evade an enemy and then use the book to play it from your discard and get out of there, which is fun.
Drawn to the Flame - This is definitely one of the more powerful cards to play repeatedly. Unfortunately you dont get to make use of the resource discount here but getting 2 testless clues multiple times in a scenario is very powerful and may often be worth losing a turn if the shroud of the location is particularly high, or the punishment for failing a test particularly severe.
Moonlight Ritual - I mentioned this above but this feels like a bit of freebie include if you are planning on going ham with the book.
Recharge - Hey now we're getting somewhere, what's better than extending the shelf-life of your spells once? Doing it twice! My feeling is that if you want Recharge you probably want it multiple times. Obviously if the first use discards your spell then you might not have a target for the second one, which does mean this card has diminishing returns somewhat. I think there's also the slight cross purpose where Recharge wants you to have use spell assets but Mysterious Vermin wants spell events, but I think this balance can be negotiated.
Storm of Spirits - Usual caveats about AoOs apply here, though this is one that you are actually a lot more likely to be using on a monster engaged with someone else, since you likely want to use it when there's multiple enemies in play.
"I've got a plan!" - AoOs etc. this card is great for helping out with boss killing, though it does come at a hefty resource cost so playing it a bunch of times might not always be viable. When you do have the money for it though this will potentially threaten 8 damage from one card which is Not Bad.
Cryptic Research, Preposterous Sketches, Preposterous Sketches - Probably only worth it during the midnight hour but 3 cards for an action is a perfectly good rate, and if you're building around Mysterious Vermin then it can help you to find it in the first place.
Deciphered Reality - Yeah so this is good. When I talked about cards that are worth losing a turn to replay, this is basically what I was thinking about. Of course you need 7 resources to play this twice which is not pocket change, but the payoff is 2 clues from every location in the game, which is obviously ridiculous.
Logical Reasoning - Same basic points apply to this as to Emergency Aid, when it saves someone from death its usually worth a turn, and its fine to chuck out on the Midnight Hour just to get a use in.
Persuasion - AoOs etc. This card is already scenario dependent, if there are lots of humanoids its usually pretty good. That means you'll know if you want this card, and if you do you may well want a way to repeatedly play it.
Truth from Fiction - So I wrote a review for this card that needs updating, but basically its an archetype card. If you're playing the secrets deck its good, and its probably good multiple times. Incidentally, the investigator for the Secrets deck is most likely Daisy who really likes this tome so hey.
Working a Hunch - Again, this card is likely going to be in a lot of decks anyway. Even at the cost of an action a test-less clue is often worthwhile. It's probably not worth a doom, but its flexible enough that you'll usually be able to fire it on the midnight hour anyway.
Alter Fate - It's a little tricky to make this work, it's not reactive in the same way as Ward of Protection that makes it totally unusable, but it is still dependent on the encounter cards you draw to be useful. You wont be saving as many actions as you usually would using Alter Fate, and you may have to pay a doom too as you cant rely on drawing those encounter cards during the midnight hour. Overall its got a low floor and a fairly low ceiling, but in some campaigns every investigator that can take Alter Fate will take it and its sometimes going to be useful to recur it.
In terms of investigators that can play this card, Item and Tome traits dont open this up at all so far so its only characters with default access to Mystic level 2 cards. I think the noteworthy ones are:
Jim Culver who's broad access to level 0 cards makes him a fairly interesting pick. Of course, he can only pick 5 but perhaps a relic Jim with Unearth the Ancients could be fun, of course he has to find a way to pass that test so maybe not... plus the Mystic relic pool is fairly handsy which competes with the book for slots.
Marie Lambeau. Marie loves this card, not only does she have access to a lot of the better cards to use with it but it also gives her another reliable and powerful way to get doom in play to activate her ability. I think this card is likely to become a staple of Marie decks, and may skew her deck building a bit more towards spell events for her ability.
Daisy Walker another home run Tome for Daisy in a cycle that's been full of them. Daisy has access to Deciphered Reality which is probably the most powerful thing you can do with this card. She also is the poster girl for the secrets deck, meaning she might want this card for Truth from Fiction. In general though, her card pool probably has the largest variety of strong insight and spell events, and now she can play them with her Tome action.
Norman Withers its a weird one for Norman, he in theory has access to some strong cards to use with it but he's very xp hungry when it comes to mystic cards so its hard to imagine him having any spare for this. Unlike Daisy he has to pay the full cost for the action too.
Sefina Rousseau she runs a lot of events, many of which will work with De Vermis Mysteriis. She doesn't get any toys from rogue deckbuilding though so she's restricted just to the mystic ones. However, all the mystic events that work with this card are level 0-2 so she really has the same access as the pure mystics, with an additional incentive to load up on events. I think this card may well find a home in some more mystic focused Sefina decks.
Lola Hayes, so you do actually have to be in the class of the event you are playing in order to use this, which again restricts it just to Mystic cards. But again, this means Lola has basically the same access as main class mystics so maybe thats not so bad. Plus, discarding a tome full of doom to her weakness makes it not feel so bad.
Luke Robinson, the spoiled investigator from The Dream Eaters also looks like he might want to glance in this cards direction.
I think in conclusion, this card is clearly very powerful in its effect but the significant costs and restrictions on it limit it to specific decks and specific investigators. I think the power of this card is only going to grow as the number of generically powerful insight and spell events grows.