[Satoru Gojo]

Hmm if it was a whole series allowed and not just a single book, it would probably be some long epic fantasy series like Malazan.

But if it’s just one? Then maybe a language dictionary so I’d have something to keep my mind on. Even if my grammar sucks, I’d at least straight up memorize a lot of vocab which would be helpful.

[Spookyy]

The Bible. I would put my treasures in heaven as were called to do.

[El_Stig]

The entire series of 'for Dummies' books.