Skill Book

A Skill Book is any book that gives the character an increased skill level from reading the book. Reading a Skill book will increase the skill level by 1 level. It is important to note that this only occurs once, so reading the same book again will not result in increased levels.

Morrowind
In Morrowind, there are a total of 135 Skill Books. (5 for each of the Skill types).

Note that reading a skill book does not count as a crime, even if the book belongs to someone else. (However, "taking" the book does, if you are noticed doing so).