The first 5 chapters of this book exemplify science writing at its best. There is the personal, but it always relates to the science, and is interesting besides. The chapters explain what we believe we know about dinosaurs and their evolution, the kinds of evidence, even what it is like collecting fossils; the chapters also discuss how a study of embryo development can cast light on evolution. Most amazing to me (p.108) is that there is sufficient collagen protein preserved in some dinosaur fossils that you can inject them into a rabbit, thereby generate antibodies, and these antibodies recognize the corresponding chicken protein.
The last two chapters are not at the same level. Horner is interested in promoting a grand experiment: modifying embryonic chicken development so that you create a creature with the features belonging to the chicken's non-avian dinosaur ancestors. There is a lot of developmental biology which must first be mastered to accomplish this. However, at the level of detail that Horner writes, there is just not enough to say to adequately fill the 50 pages devoted to the subject. As explained I couldn't fully appreciate the concept of symmetry, and Horner did not want to go into such topics as just how staining works in tracing embryo development.
Horner himself is a delightful man, careful to give credit where it is due, willing to tell some anecdotes which do not paint him in the best light, and willing to spend $40,000 of his own money to sponsor research he believes in! However, sometimes he seems to be implicitly assuming that it is only the control genes which mutate, so that just by modifying their signals you can creat a viable creature with dinosaur characteristics.