What's the point of buying books?
Solution 1:
They are kind of like the paintings from Assassin's Creed II and Brotherhood: there is no real in-game point to them beyond filling up a room at the hideout. You can't read them or interact with them in any way other than getting some information about the book and its author.
You do get an achievement for buying all the books from every city and doing the Niccolo Polo book missions, but that's about it.