A book is a set of bundled pages, usually paper or parchment, usually bound together by a cover of some sort. It is often a written source of information or entertainment.
A book can have content that will tell your character more about the lore, about the history of certain places or legends, and so on. In this sense, it will most likely be interesting side information, although it will sometimes give a character the necessary hints to continue with their quest.
A book can contain written stories, legends or tales, that are not real most of the time. As for storybooks and novels, a character might carry them around for the emotional value, such as childhood memories, that is attached to them.
Apart from the prior mentioned purposes, a book might also be used to learn skills such as languages or other theoretical skills, spells, and more. Even practical skills such as swordmanship can sometimes be studies with a book.