Pact of the Tome is a Pact Boon from the Warlock class. You receive a book that allows you to cast several spells.
Your patron grants you a grimoire called "The Book of Shadows", which allows you to cast 
Guidance, 
Vicious Mockery, and 
Thorn Whip.
At level 5, Warlocks gain 
Deepened Pact which grants more spells:
Classes:
