One of the four booklets in the main Warhammer Fantasy Roleplay box set. From the publisher:

Warhammer Fantasy Roleplay is a new, exciting way to experience the popular Warhammer Fantasy setting. It is a grim world, constantly at war. As a hero, you will take up weapon, spell, or prayer and do your best to combat the might of enemies terrifying to behold. As the Game Master, you will make the lands of the Old World real as you craft the story, the people, creatures, and the mysteries the other players will encounter during their adventures.

The Tome of Blessings provides a brief history of religion in the Empire, and describes the major faiths practised in the setting. This book provides additional game rules and mechanics for invoking divine blessings, as well as useful information for players who want to play a priest or religious character.