Quite informative, from both a gaming and storyline standbpoints. You get both gameplay and novel-like mechanics for a setting, whether it's the refugees of Bligdenstone, or the Gold Dwarves of the Great Rift, or the Uthgardt Tribesfolk of the North.
If you're going to be getting involved w/ 3.5 Edition FR concepts, I'd suggest
Players Guide to Faerun for 3.5 Edition updates to various material. Think of it as a patch if the books all were a PC game.
Magic of Faerun details a number of spells that appear in PGtF, as well as info on spellbooks, various magic items & materials, gems (particularly in relation to crafting wands), info on different types of magic, and the famed Magister Template. There are some other things, but that might make quite a list.
For information on past civilizations and playing campaigns in ruins of cities, I highly suggest
Lost Empires of Faerun.
Faiths & Pantheons details the existing deities of the Realms, along with how they and their Clerics behave and interact with the world. You'll find stats for a good number of the deities, particularly of the Faerunian/generic human Pantheon. Unfortunately, the only demihuman deity to really get statted in that book is Lloth (The drow have a fanbase around the size of Star Trek, no?), but I know WotC's FR board has a stickied thread of deity stats. Those who didn't get their regular profile blocks & Cleric-specific data noted in their entries.