If you're looking at this book, you probably already know what you're in for. This is a tremendously detailed text on comparative morphology that seeks to trace the origins of organ systems from the original metazoans on downward. Each chapter takes a separate organ system -- nervous, circulatory, excretory, and so forth -- and discusses how it is structured with respect to the phyla (ALL of the phyla). It then concludes with the author's analysis of the implications of the morphological discussion for evolution -- which features were original to a shared ancestor (thereby indicating relationships between the phyla), and which are instead likely just the product of convergent evolution.
Not being an introductory text, this book will probably be incomprehensible if you do not already know a fair bit about all the metazoan phyla and some of the competing theories regarding their origins and differentiation. But that being said, it's not overly complex otherwise, and is a very interesting read. The illustrations, moreover, are well done and plentiful.
If I had a criticism, it would be that the text gets a little dull and rote, mechanically following its organizational scheme without as many digressions and charming anecdotes as I'd like. But you can't have everything. There's really no other book out there in English, to my knowledge, that is as detailed on this subject. So if you are at all interested in comparative morphology of the phyla, this is the one for you.