Spellbooks were used in Diablo I to learn Spells. They could be found in various places in the Labrinyth such as Bookcases, Skeleton Tomes, and Chests. In addition, they could be obtained by killing enemies or purchased from Adria. They teach you spells of whatever the book's title was. For example: A Book of Firebolt teaches you the Firebolt spell. If you happen to find the book again, you can learn the higher leveled version of the spell. There is a requirement of the level of Magic you must have before being able to use the Spellbook, the higher leveled spell it teaches you, the higher the Requirements it will have.

Here are the all the available spells you can learn:

In Diablo II and Diablo III, the spell system was replaced by the Skill Tree, eliminating the need for books. Instead of using books you would have to spend skill points on Skill Tree in order to make your spells/skills more effective.