Spellbooks were used in Diablo I to learn Spells. They could be found in various places in the Labyrinth 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.
Not all who wish to improve their spells through reading books may have a chance, however. There is a enforced requirement on a hero's Magic attribute. If the hero does not possess that requirement, they are unable to use the spellbook. For each level earned for a spell, the Magic requirement to gain another level increases by 20% until the requirement reaches at least 192 Magic. After that, the next level (and all subsequent levels, if applicable) require a Magic attribute of 255. The only way to get by without such an imposing restriction is to make use of certain shrines, and even then, all of those shrines make the hero suffer from a tradeoff, which most of the time may be too harsh for trivial gains.
Here are the all the available spells you can learn:
|Spell||Initial Cost||Cost decrease /level||Minimum cost|
In Diablo II and Diablo III, the spell system was replaced by the Skill Trees, eliminating the need for books. Instead of using books, you would have to spend skill points / choose the desired skills and runes from the list on Skill Trees in order to make your spells/skills more effective.