I just got finished reading Sean Carroll's Endless Forms Most Beautiful, and I thought I'd give my thoughts on it. This book is a little more complex than Your inner fish, so I would not recommend it to a complete science beginner. For this book, I think it is better to have a bit of a background in science, particularly in genetics. The book does try to use simplifying language to explain some of the genetic concepts, but I personally found it more confusing, and found my self having to stop and think about what the correct terminology was to understand it.
That being said, it is a fascinating book, that goes a bit more in depth about evo-devo. For me, I most enjoyed the talk about the genes behind wing formation, as one of the discussions I have with my students is about determining if insect wings are purely outgrowths of the exoskeleton. But even more fascinating was how all of these genes are regulated at the 'beginning'. Now the genes are turned on or off, by the presence or absences of certain proteins. These proteins are made be genes that are turned on by the presence or absence of other proteins, and so on and so forth. So what creates the gradient conditions of proteins to turn on the initial proteins?
Carroll puts for the idea that this may be due to uneven deposition of nutrients in the egg. I find this idea interesting and wonder if it has actually been looked at. Is this unevenness repeated in every egg laid down by the mother? Or caused by the first division or subsequent ones? Is there a difference in the 'unevenness' causation or how the initial genes are turned on between organisms which experience determinate vs. indeterminate cleavage?
So for me, the book was most interesting in the additional questions it raised. It also showcased just what one can do in the evo-devo field, and how that relates to variety of other disciples, including paleontology. So if you're a bit more curious about development and genetics, I would recommend this book.