The book tries to establish a set of rules for REST interfaces - I'm OK with that, it's what its title says. When you define rules, you take an opinion, that's fine. And most of what is defined as a rule is something I'd support as a good practice. A possible benefit of this rule book would be to provide some compact, memorizable, and clear guidance on good practices.

But then there's this obsession about WRML.

WRML is a proposed way of defining the content types and schemas of REST APIs - see http://www.wrml.org/about. And what do we find as the sole literature on WRML: the REST API Design Rulebook. WRML has about zero industry acceptance; I never heard of it, and it does not even have a wikipedia page. And yet the REST API Design Rulebook puts it into the heart and center of every halfway advanced topic. Now a "rule" that is centered around an obscure standard proposal is totally void, and so this book is a simple waste of e-ink (even if that was its only fault, which it isn't).