The Inner Bird is not a typical bird book, and therein lies a great deal of its value. It is the first recent popular science treatment of the anatomy of birds, and how that anatomy ties into the features more commonly written about aspects of avian biology; evolution, behaviour and taxonomy. Even experts of bird ecology or taxonomy will find something of value here, it brings back the almost Victorian emphasis on anatomy, on the skeleton, on how the shape drives the function (and visa versa). The treatment of evolution takes the recent revolution in our understanding of birds as dinosaurs from a skeletal point of view, and the recent breakthroughs in avian systematics and cladistics as derived from DNA and molecular analysis are explained for the layman and contrasted with previous attempts to assemble a tree of life for the class Aves.
The scholarship isn't always flawless, there are a few statements that will leave you scratching your head ("[bee-eaters] are one of only a few small non-passerine birds that undertake lengthy migrations"?). The price is also rather hefty, and the index could have been more helpful, but these are minor distractions in what is otherwise an excellent book that fills a gap in the market.