Deities & Demigods (3rd printing), hardback core rulebook for AD&D 1st edition, by James M. Ward and Robert J. Kuntz
“Deities & Demigods Cyclopedia provides the Dungeon Master with gods, heroes and monsters from myth, fiction and legend for use in rounding out an Advanced D&D campaign.
Within this book are fifteen pantheons of divinities, each profusely illustrated. Also included are new material on clerics’ conduct and their relationships with their deities, information on character mortality and immortality, and more!”
Note that this is a third printing. That means that the Cthulhu Mythos and Elric of Melnibone sections have been removed (although there is still a note thanking Chaosium for allowing their inclusion). See here for a full explanation.