There are many books in Tibia. Their content range all from just a title, to clues for quests, to full length stories spawning more than just one book. You can usually find books in libraries in towns, but sometimes also near ruins of old houses, in dead trees or barrels out in the wilderness. These do often have a clue for a quest or how to find a place, item or key.
There is also a type of book that is magical. They are called spellbooks. The magic thing about them is that their content changes depending on who is reading it. The content always tells which spells the person reading it knows and some information about them.
Some types of empty books that you can write in can be bought in shops, others can only be found as loot from certain creatures. Note that some writeable items are not books (see Documents and Papers).