I read the Origin of Birds by Gerhard Heilmann for the decades challenge. This book was published in the US in 1927 and was very much ahead of its time in its content. It is more than anything else a book about the similarities between birds and reptiles and makes one of the stronger arguments I've ever read for common ancestry between these groups. Long before Velociraptor and Deinonychus and other very famous bird-like dinos were found, Heilmann saw the important similarities. He found, as have later researchers, so many similarities between certain reptiles and birds that they MUST be related. My students are to this day absolutely astounded to hear of this, but it was noted as soon as Archaeopteryx was found in the 1860s. This book has a wonderful description of Archaepteryx.
It is an incredibly detailed book. The level of anatomical detail would probably bore most people without a vested interest in vertebrate zoology to tears. But I found it extremely educational. Heilmann, as I stated earlier, was a great arguer. His rebuttal of the hypothesis that birds are related to bird-hipped Ornithischian dinosaurs was scathing.
This is not an easy book to get one's hands on. I managed to buy an old library copy for only $37, but some copies approach $100. Glad I got it and glad I read it.