Faiths and Avatars, Powers and Pantheons, and Demihuman Deities make up for one of the BEST Forgotten Realms accessories (they are in trilogy form) ever made. The Trilogy sheds light on exactly what the title says: Gods-and there sure is lots of them! The books, which are very well written and very well presented, also focus on other areas such as each church's distinct culture, tradition, and history. As a result, they will win you over for sure!
Eric Boyd does a FANTASTIC job of presenting the different pantheons, ranging from background history, to rank descriptions, to their respective magic, thus allowing for the creation of important people and characters, to adventure hooks and encounter tables, to magical items and artifacts, to new spells and treasures. This accessory has it all and more!
Each god has his own supplementary text information, clerics' and worshippers' alignments, Specialty Priest class and their alignments, church symbols, specific spells, special abilities, dress garb, even information relating to actual temples.
For other FR references/adventures, I STRONGLY recommend: the Old Empires accessory on Chessenta, Mulhorand, and Unther, Dreams of Red Wizards on Thay, Dwarves Deep, Draconomicon, the Jungles of Chult and Moonsea accessories, the Shadowdale, Tantras, and Waterdeep adventures, the Ruins of Myth Drannor (Elven pantheon) and the Ruins of Zhentil Keep Box Sets for Bane vs Cyric material (they are Second Edition AD&D, out of print and it will take a bit of searching, but it's well worth it). For updated editions of events in the Realms, see the Third Edition Forgotten Realms Campaign Setting, which even though is quite expensive, is still very useful to all FR fans.
Moreover, for those interested in the gods, the Forgotten Realms novels to read are: The Avatar Trilogy- Shadowdale, Tantras, and Waterdeep, the Prince of Lies, and Crucible: the Trial of Cyric the Mad. In addition, the Ring of Winter is relevant to the Chultan pantheon and specifically to Ubtao, as it is the only novel set in the Jungles of Chult.
Faiths and Avatars, Powers and Pantheons, and Demihuman Deities along with the Forgotten Realms Adventures accessory, and the three Forgotten Realms Campaign Settings (one for each edition) are a "must!" They all compliment each other.
Whether you are playing in the Forgotten Realms or in the Planes, this trilogy of books will be of tremendous help in collecting all the information you'll ever need.