Summarised from the Introduction:
Monster Mythology is a companion volume to AD&D 2nd Edition Legends & Lore. Non-human pantheons were not included in Legends and Lore, and this book makes good this deficit. Monster Mythology is of greatest use to readers who possess Legends and Lore, but it is also usable by those who do not. A summary of important general rules and notes from Legends and Lore is given and expanded upon here.
Monster Mythology has a similar style of presentation to AD&D 2nd Edition Legends and Lore. The revised Legends and Lore was a complete rewrite of the original, with an emphasis placed upon role-playing and using the gods (and their avatars and servants) in a fantasy campaign. Likewise each section of Monster Mythology describes the cultures, societies and myths of the races who worship the pantheons in question.