"Books" (like the "Book of Orcs") will tell you about a specifc monster. Knowing things like their number of hit points will help you decide whether to fight them and which weapon to use. A book is not really very useful to carry around though, unless you like the next tip....
Carry some Straw around. You will find some. You can't do anything with it, but it is nice to use to mark your way or to mark places you've been. Likewise, you could leave a "Book Of Orcs" lying near a doorway if you think you are going to get lost!