Purpose of Books?

[Shai-Hulud]

In Gothic 2, many traders sell books, do they do anything, or are they just for lore? With the exception of the 2 the lutero sells and the one from Constantino, they are not mentioned in any guide

[D'amarr from Darshiva]

There are a couple of books that give various amounts XP or LP.

[Qju]

the only books that are availeble are the two books from Lutero wich both give you a bonus for your fighting skills either with one-handed or two-handed weapons. Then there is a book called "Divine Power Of The Stars" it gives you a mana bonus but only once.
There are other books in the game but not for sale. Most of them are attached to a quest or gives you information about events or charakters.
If you have any other books in your game, than it's because you have a modifikation installed.