Books in roguelikes are mainly used as sources of spells to be learned, although they can have other uses, such as being readable for special information ("plot" or "world" data) or providing training (as with Crawl's manuals or Steamband's skill point granting books).
A single book may contain only a single spell or it may contain a collection of spells. Studying may (eventually) consume the book or it may be a durable item. Destruction by studying generally only makes sense if one gains only a limited knowledge of the spell(s) studied with each use. Most games allow you to study a spell, then drop the book, but Moria and Angband require that you have the book available to read from in order to cast the spell.
The most important function of spell books in most roguelikes is to control spell availability. They serve other functions and are not the only way to restrict spell availability, but they are a tested method and many possible variations on the spell book theme remain to be explored.