Of all the monster manuals I've gone through in my time (and believe me, being a GM for 20 years provides you with a healthy load of those) Out of the Pit (OOTP) is beyond doubt the one closest to my heart. Running from the earliest black books of AD&D, all the way up to Bestiary 3 from Paizo, nothing leaves me with the same thrill and sense of fantastic beings as this one.
A huge issue I often take with modern monster books (for RPG's at least) is how too much is lost to cold, hard facts and not enough fluff. Sure, it's nice to know that the black spawn from Hell lashes at you with four 2D8+2 attacks, and that it is able to shoot fire from its ears. But where does it live, how much does it sleep and how many kids does it typically have?
If these questions tend to bother you the same way as they bother me, you'll love OOTP. Unlike many modern monster manuals, which have a tendency to stick with evil and threatening monsters, this volume carries high the old Fighting Fantasy tradition of including strange and somewhat just odd creatures. The Jib-Jib comes to mind, among others.
It's this free spirit; the will and courage to actually play with your creations, and include trivial information about them, that makes this book stand out to me. It's a grand tour down nostalgia lane if you ever were much into Fighting Fantasy, and/or like me included them in your homemade campaigns for your friends. Even then, I remember having a great time keeping it around as a reference, whenever I played the solo adventures in Fighting Fantasy.
For some people, this book has become some of a collector's item and I deeply recommend getting it, if you have the chance. It's a funny little read, from when roleplaying games and fantasy didn't take itself all too seriously.