This is one I found quite cheap on the shelves of one of my favorite local book stores while out with my best friend. He taught me D&D and has probably forgotten more about this hobby than I'll ever know, so when he immediately called me over and told me to pick it up, I did so.
This book hails from the 3rd edition ruleset of D&D, and is a book that lays out the setting as it was at that point in its timeline for the Forgotten Realms. The Realms were created by Ed Greenwood back in the '80s and became so popular that they effectively became the default setting for D&D games. That's all but officially become the case with 5th edition, so reading this book was a bit of a jolt as it occurred to me that there used to be a lot more settings for D&D, so the Forgotten Realms needed updating and fleshing out for this new ruleset.
This is just a great book. It gets so deeply into the nitty gritty of the Realms that it even lists imports and exports for all the major countries, states, etc that it expounds upon in the book. It has a few pieces of great art (less than I'd like, honestly), two great little one-to-two shot adventures, and a bevy of information about the politics, religion and other movers and shakers in the Realms. I highly recommend this book for any DMs looking to write their own stories in this setting as one of 5E's only true great flaws in my estimation is a lack of dedicated lore books, but there a few caveats. First, you'll probably want to read this with the Forgotten Realms wiki open as many things about the events, characters and locations listed here have changed in the 25 years since this book was written. Second, finding it will likely be a bit tough because, as per a bit of research I did about a month or so ago, this book is running anywhere between $50 to $100 even on the secondhand market.