Spell book

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Spell books contain spells that can be memorised, if you have sufficient skills in Spellcasting and the school to which the spell belongs. They are very important, as they are the method by which spellcasters gain new spells. Spell books can be found on the Dungeon floor, bought from shops, given as gifts by certain gods, or generated with a scroll of acquirement.

Spell Book Identification and Use

Most of the time, reading a spell book is enough to identify it, revealing the spells it contains. Once a spell book is identified, you can attempt to memorize any spells it contains, provided you have free spell levels and high enough magical skills to handle the spell in question.

However, a few powerful spell books require you to have certain magical skills, or they will be "beyond your level of understanding," preventing you from identifying them:

The Book of Annihilations requires either level 6 in Spellcasting and level 10 in Conjurations, or to be a follower of Vehumet.
The Grand Grimoire requires either level 6 in Spellcasting and level 10 in Summonings, or to be a follower of Vehumet.
The Necronomicon requires either level 6 in Spellcasting and level 10 in Necromancy, or to be a follower of Kikubaaqudgha.

For followers of Trog, identifying a spell book will decrease the piety gained from burning it.

Spell Book Destruction

Advanced magic users often need to change their spellsets. Scrolls of amnesia allow them to forget any spell and recoup the spell levels to learn a different spell. However, these scrolls are rather rare. Conveniently, spellbooks can also be used to this effect, however you are limited to forgetting one spell contained therein, and the process destroys the spellbook itself. (Followers of Sif Muna never need to do this because they are granted this ability after sufficient piety.)

Strategy

Spell books are not exactly rare, but there are a great many of them. They're also rather expensive in shops. Thus, obtaining a specific spell book that you want can be a hassle, but you can alleviate this by worshiping the right god.

Sif Muna and Kikubaaqudgha both grant spell books as gifts, weighted towards spells/books you don't already have. Sif Muna is the most generous in this regard: since the types of spellbooks granted depend on magical skills, you can influence them by training specific skills, and thus raise your probability of getting the most useful spells for your playstyle. Followers of Sif Muna will eventually have access to every spell in the game this way. Meanwhile, Kiku only grants Necromancy-related books (including the Necronomicon at the final piety level), making him in some ways better suited for Necromancers than other spellcasters; on the other hand, he grants two randart books as you gain piety, containing level 1 to 3 necromancy spells at the 1* piety level and level 4-6 necromancy spells at the 3* piety level, so he's a good choice for training up Necromancy from scratch. It is also worth noting that Kiku starts granting his books much sooner than Sif Muna.

Note that you keep your gift books even if you abandon your god. Some advanced players take advantage of this, serving Kiku until they get the Necronomicon (and can survive Kiku's divine retribution), then leaving Kiku for another god who offers further powers.

Followers of Trog, who shun spellcasting, can burn any visible spell book as an ability. This not only scores piety, but the flame cloud generated will remain for several turns afterward, potentially blocking or damaging monsters (especially if they were standing on the book when you detonate it). Note that no monsters avoid spell books on the floor or see them as a hazard at all, making it easy for you to use your books like land mines.

List of spells by book

Main article: List of spells by book

The following are all standard spellbooks in Crawl. If you find any book other than the ones listed below (often with exotic or ridiculous names), it is a randomly generated artefact spellbook. Randart spellbooks have no set contents but may often contain just the spell you were looking for.
Please do not add randart spell books to this list.