This was fun to flip through, because it can be tough to find books about extinct animals other than dinosaurs. But apart from the focus of its subject and well-designed layout, it's disappointing.
The entries themselves are badly written. There are lots of short, declarative sentences piled up on each other, which come across as boring and monotonous. And when the author does indulge in different types of sentences, they're sometimes awkward or unclear.
I was also annoyed by the book's tone and some of the conclusions it presented. For example, it claims that the large number of dire wolf fossils found in the La Brea tar pits means that dire wolves "must have been very numerous animals, very stupid, or overly aggressive." Really? Those are the only possibilities? There are lots of little, unsupported judgements like this, which made me doubt the quality of the rest of the research.
I wanted to rate this more highly just for being a book about megafauna, but it's not worth its hefty textbook price tag. I'm glad that I was able to borrow a copy.