So... pretty early to post a review of a card that won't come out for probably a long time (if ever), but I'm pretty hype for this one.
The big thing for me is that normally investigators tend to rely upon hand slots for boosts, whether it be from the classic Mag Glasses or the recently fairly popular Hawk-Eye Folding Camera. That means that if you're running cool cards like Occult Lexicon you sacrifice some ability to boost your . Whitton not only helps alleviate this problem by giving you a boost while you're holding a tome or relic, but also helps you find said tomes and relics.
The obvious application is Daisy Walker, as she is so far the only investigator for whom (most) tomes don't suck. However, the first thing that came to my mind was Ursula Downs with the Ornate Bow (which is a relic). Normally you have to sacrifice some investigative power in order to wield the Bow (unless you can get Elli to wield it for you, but when I've gone the Elli route in the past, I usually end up cutting all other hand slots anyway because it's so unlikely that she'll find the Bow, specifically). I think it's also worth mentioning that I almost always run Crystalline Elder Sign in Ursula, especially in true solo. Whitton can also help you find that! To be fair, between Crystalline Elder Sign and
Fieldwork, I seldom feel like I need help investigating, but there's always stuff like The Last King where most locations don't have clues but you still want boosts (Fieldwork can't do this, though neither can Mag Glass). And it certainly never hurts to be 1 higher in intellect.
Another point is that you need to put into play or reveal a new location to trigger it. Having played with Jake Williams before, I can tell you that this is better in some scenarios than others. The Depths of Yoth and The Boundary Beyond in particular have a lot of this, while other scenarios start with all locations revealed, or just don't have many locations. Still, in true solo, at least, you'll probably be revealing a lot of locations, and each time you get to search for a relic or tome. Makes it much more likely that you'll get your key assets out early.
Also, you can't deny that having 2 health and 2 sanity is quite a bit better than Milan's 1 health and 2 sanity.
Some other notable relic traited assets that Whitton Green can help you find: Ancient Stone, Key of Ys, Grotesque Statue, Pnakotic Manuscripts, Hallowed Mirror and many others.
I am less knowledgeable about what tomes are good, but definitely Encyclopedia and Old Book of Lore for Daisy, and eventually The Necronomicon (Petrus De Dacia Translation) (even for non-Daisy). Also Occult Lexicon, which I'm a pretty big fan of in a lot of decks (especially ones that don't use the Bow).