In the PHB, a book has a listed description and value (25pg), and in the scholars's pack a book of lore is listed while those with the cloistered scholar background (SCAG) start with a book on a subject of your current study. Nowhere is it listed what exactly these books are intended to provide players.
In our current campaign, there really isn't anyone churning out books on printing presses, so books are hand copied and not exactly plentiful. I want place books in loot for players to acquire and occasionally give them opportunities to get books from merchants or collectors. As they will be in loot and in shops, I want them to have some intrinsic value to a clever character other than just something to sell in town. For the most part, I'd like to stay away from using them just as a plot hook, as those sorts of books a player would have to go searching for, not fall into their lap via a treasure hoard. I know that I want the books to interact with skill checks in some way, however, I am not sure how useful I can make them without them becoming unbalancing.
What sorts of uses for such a book are appropriate, yet on balance for their value (25gp and up) and rarity (established above)?