Spell book is an object where heroes store their learned spells. It is also needed to cast a spell. Any hero can have a spell book, but typically might heroes (e.g. Knights or Beastmasters) do not have them at the beginning of custom scenarios; however, Alchemists and Death Knights do both start with a spell book.
Spell books can be bought from a mage guild for 500 or obtained by equipping Titan's Thunder combination artifact. In advanced maps it is possible to find a spell book placed as an artifact on the adventure map, but it is not allowed in the default map editor. A spell book cannot be traded to another hero, nor can it be lost after a defeat.
In the Shadow of Death campaign "Birth of a Barbarian" Yog the Wizard cannot ever acquire a spellbook (entering a Mage Guild while he is a visiting Hero results in a messagebox telling "Yog has given up magic in all its forms. His knowledge of battle and tactics will have to be enough to get him through the rest of his life.").