Telescopes, Tides, and Tactics: A Galilean Dialogue about The Starry Messenger and Systems of the World - Stillman Drake
This is a really nice way to accompany the text of the Starry Messenger with discussions of the methods Galileo used to make the observations; errors and their causes; follow-up observations that Galileo made after the book was hastily published; and contemporaries' responses to it. The "dialogue" is a little stilted, but really not bad at all -- the "participants" seem human enough, and even though they don't disagree with each other as in most "dialogues", they do ask for clarification and chip in alternate suggestions, which helps to "lighten" the reading.

I have to admit that I didn't take the trouble to work through some of the arguments in detail: I'd just say to myself, "Here follows a trigonometric demonstration of argument Z, let me take Z for granted." I also skimmed all the observations of Jupiter's positions. However, I did appreciate reading through how Galileo showed that the glow seen on the dark parts of the Moon is reflected light from the Earth. Turns out, too, that this is a contradiction of medieval cosmology, which would have assumed that the earth did not shed light. Also, even though I didn't fully understand the proof, it was interesting to learn that the phenomenon of the eclipses of the moons of Jupiter was a disproof of Ptolemy's astronomy, like the better-known phases of Venus; no wonder that the latter gets more publicity, since it's more easily understood by a layperson.

One of the topics that Drake hopes to address in this book is why many philosophers refused to look through Galileo's telescope: "...a mere optical phenomenon, not regarded by most philosophers as even relevant to a book about so sublime and grave a subject as the system of the universe, and hardly worthy of consideration except by mere mathematicians. Occasional and incidental appearances are not taken by philosophers to be proper bases from which one may reason about the structure of the world..." (However, "Aristotle... taught that a single contrary experience outweighs any amount of subtle reasoning.")

Drake really took this as a lead-up to Galileo's description of the "system of the world", so long delayed; he shows that as well as spending years collecting observations, and hypotheses derived from observations -- in particular, finding phenomena that link earthly and heavenly motion, such as the tides -- Galileo was trying to prepare the ground for the reception of his major work by establishing that it was appropriate to do science by observation at all -- a view that had all too few partisans at the time of this dialogue, 1613.