I don't buy monster books, but nor do I buy adventures, rulebooks, campaign settings, or sourcebooks. I can usually make up whatever I need, or if I really want something, I've probably written it myself.
However, a small book like that is somewhat interesting. I'm not much of a critter guy, but if the monsters had a lot of flavor, perhaps a few with mythic overtones, then I might be interested. I wouldn't buy a book that just had new things to beat PCs up with, though. I'd want something that an adventure itself could be based on.
I mean, really, how often in classic folklore do you see more than one type of monster in a single story? Even Greek heroes didn't fight all their monsters at the same time ("And suddenly, a hydra, a medusa, a sphinx, a hellhound, and a bunch of harpies attack!").