The Book of the Righteous presents the most comprehensive pantheon ever seen in the d20 System. This massive, 320-page hardback provides more than 20 pick-up-and-play churches corresponding to gods that feature in most fantasy campaigns (god of war, god of justice, etc.). Each church features lavish detail, including in-depth information on its clerics, holy orders, dogma, prayers, and rituals. These churches can be used in any campaign to bring a whole new level of detail to the religious characters. Plus, for those who don't have a complete cosmology in their game, The Book of the Righteous provides a comprehensive mythology that unifies all of the gods in the book. That mythology, and its corresponding cosmology, is not tied to an existing campaign setting, making it as portable and useful as Green Ronin's critically acclaimed Freeport series. The Book of the Righteous also features a new core class, the holy warrior, and new prestige classes, feats, spells, domains, magic items, and artifacts. Third in Green Ronin's Arcana series, The Book of the Righteous is a divine sourcebook like no other.
If you want to homebrew a setting for Dungeons and Dragons' 5th edition, this is a good resource! A good, solid foundation for a panteon if you want a pre-written one, a lot of info on how to adjust the panteon to your particular setting, and info on how to make your own gods and mythology from scratch. Combined with a number of fun sub-classes and new spells (naturally focused on divine matters) this is a worthwhile addition to your D&D book collection, should you have one of those.
The only downside is that this book is rather dense, text-wise. I've got poor eyesight and limited free time, so getting through this all took a good while.