Best book in the DMGR series. Monster Mythology sees more play time at my table than any other 2e book outside of the core rules. DMGR4 is necessary whenever a DM uses demihuman, humanoid, or monstrous priest NPC's, cults, temples, or creation myths in 2e.
Some DM's may prefer the later Demihuman Deities, particularly those running Forgotten Realms, but DMGR4 is superior both as a general reference and an inspirational foundation for D&D mythos:
1) author Carl Sargent's writing and creativity is top notch. The pantheon relationships and creation myths come alive. Those who enjoy the prose and detail of classic UK designers like Paul Vernon, Basil Barrett, and Graeme Morris will feel right at home.
2) The power level is perfect. Priests get a healthy amount of spheres, granted powers, and arms, but not too many for a class that hits second level at 1,500XP. Specialty priests saw continual "power creep" throughout 2e. In early works like PHBR3, Legends & Lore, and Greyhawk Adventures, priests had few spheres and were weaker than their 1e counterparts. In later 2e products like the Faiths & Avatars trilogy and The Scarlet Brotherhood sourcebook, priests have numerous spheres and powers. DM's should compare carefully and decide what's right for their game.
3) It's compact. Other sources have useful details for in-depth development, but if shelf space, quick reference, or portability are desired, DMGR4 is the best buy.
4) For 2e Greyhawk campaigns, DMGR4 is the perfect companion to the gods of the Flanaess in Carl Sargent's From the Ashes. The power levels match and both books reference the other.
[5 of 5 Stars!]