I received the book last Friday and have read most of it over the weekend. First, let me complement you on a well done job, eminently readable and very informative. As a regular at this site, I did not think I would get much new information out of the book. I was wrong. The section on eclipses was particular enlightening; for instance, I had never heard the (apocryphal) story of how Galileo went blind. Kudos.
Now for my historical nitpick. On page 99, you wrote that Einstein won his Nobel prize for his work on the photoelectric effect not his "much later work on relativity". I assume that you are referring to the general theory. (Since the intended audience probably does not know their is a difference between the special theory and the general theory, it's probably wise not to distuingish between the two there.) The special theory and the photoelectric effect/light-quanta hypothesis were developed at the same time, 1904-1905. Even the general theory and the photoelectric effect overlap a little. The last paper on the photoelectric effect was in 1909 but in 1907, he already published on the gravitational bending of light. All this boils down to, leave out the word "much".
The previous paragraphs describing how Plank developed his theory of blackbody radiation are very good. It's difficult to simplify and make the development understandable to the layman without making it so simple the importance is lost. I think you did good job of striking that balance.