I'm not saying she'll imediately turn evil, I'm saying 'don't teach a teenager with an evil staff-mentor that constantly urges on her worst impulses how to do high-grade black magic'.
Then you're saying that she'll slowly turn evil? Because that is the only logical consequence I can read from what you're saying.
I don't think that would happen, because although Janna is a prankster she's also a good person and she'll know when she's going too far. An example, stealing Doof's wallet is a pretty nice joke, putting a curse on Doof so he's terrible nightmares for a week straight, not so nice. In fact that not so nice example seems like something she'd have been able to do already with Felldrake assistance if she really wanted, however she hasn't so far.
In any case, even if that were the case (which I don't think it'll be) we could talk to her or psychoanalyze her if needed so she realizes that what she'd been doing was wrong.
I wouldn't even worry about the book.
I'm sure we'll get a description of what the book does or the research we can do on it.
It may have some hidden stats or something but oh well.
The gms aren't going to try and deliberately screw us over.
If the book eventually will do something bad to us I'm sure it will be spelled out for us in multiple interludes.
I completely agree with this.
To add a bit more to this, on a meta-level, the book was an item we could have bet in the auction and that Janna really wanted. Just imagine that we've gone for it instead of the Talisman and then the moment we used it on Janna she starts getting evil, it'd be a complete dick move on the qms part if using it on her suddenly made her start to go evil.
Also, a bit unrelated to the book but going back to Janna sheet I realized this:
Lord Feldrake: This birdbrained magical staff claims to have once been a universe ruling dark wizard. Whether that's true or not, His magical power and knowledge is granted to anyone willing to wield him and further his dark designs! While his full potential has not yet been unlocked, he currently grants +12 to the Occult of whoever wields him. Since he is immobile and lacks focus (or rather, everything but a focus), Feldrake cannot betray you without the support of his wielder.
Feldrake can be reassigned, but beware! He will only agree to change to a new hero unit that is either more powerful or evil-er than his last!
Does anyone have any good idea about how to unlock this? Personally, think this may be locked behind interactions between Felldrake and its (or his, dunno what pronoun use here really) wielder. The other option would be that it's locked behind Felldrake's World Tour, but given that he admitted to not having been to any of those places I find this a less likely option.