This is a nice little collection of monsters for the Basic edition of Dungeons and Dragons, though the concepts, if not the exact mechanics, could easily be used in plenty of fantasy games. The book starts with a brief discussion on how to vary monsters, with ideas like using different names, changing up their abilities a bit, and making environmental versions, like the classic video game RPG thing of the fire, ice, water, etc. versions of a monster. The main way this manifests in the book proper is that there's a whole bunch of underwater versions of normal monsters, from undersea beholders to aquatic vampires. The latter have a neat twist of dying when immersed completely in open air, but in general I don't have a huge interest in underwater adventuring and thus find lists of water monsters a bit dull.
There's also a number of new low-level humanoids, some of which are neat but run into the problem of being mechanically kinda samey. Plus, there are no standard rules for using monsters as PC races like in later editions, so I can't easily fulfill my dream of playing as a turtle person or the two different kinds of cat headed humanoids. There's also a fair number of giant versions of normal animals (though they do suggest to me an adventure focused around giants going hawking) and weird death trap monsters. There are a number of monsters that I do like, such as the epically powerful earthquake beetle, the faedorne (in which elven havens are above the world not across the sea), the albino shadow elves, and others. At least a few monsters suggest potential adventures or at least interesting encounters, which is nice. The art is entirely black and white and not the best I've seen, but there are some good full page pieces and it generally does a fair job of depicting the various monsters.
I think in general this book runs into the problem common to later Monster Manuals: all the most iconic monsters, as well as those unique to Basic, are covered in the Rules Cyclopedia, so this is left with some interesting stuff as well as a lot of dregs pulled in from various sources to fill out the page count. There are enough neat monsters to make this a fun read, however, and even things like the generic humanoids and giant animals could be fun to use on occasion. Plus the book includes expanded encounter tables with monsters from itself and the Rules Cyclopedia and an index to all Basic D&D monsters, which is pretty cool. I feel like this is nowhere near as essential as the Rules Cyclopedia or your old school core book of choice, but it's a nice source of extra monsters and adventures ideas and I don't mind having it. Besides, it's always fun to read about new monsters and the story ideas they give.