Monsters of Faerun
Monsters of Faerun was the first hardback Splatbook written for Dungeons and Dragons Third edition. It was preceded by three other splatbooks - the Living Greyhawk Gazetteer, Hero Builder's Guidebook, and Sword and Fist - but they were all paperback.
It's basically a second Monster Manual, written before WotC realized that they could just keep adding Roman numerals after the words "Monster Manual" instead of giving each one its own unique special snowflake name. Despite focusing on creatures from Forgotten Realms, it is not, technically, a Forgotten Realms book; The first of those (for 3e, anyway) would not be published for another four months. It includes somewhere around 140 to 150 monsters, and did not give any of them PC stat blocks or level adjustments; however, quite a lot of them would be reprinted as PC races in later splatbooks.