This is a collection of ten what-if scenarios for alternate earths in various solar systems. It includes the title scenario of Earth having two moons, how we would have gotten them, their effects on the Earth over time, and ultimately what’s going to happen to them. Other scenarios include the Earth as a moon, the Moon in a retrograde orbit, other planets in Earth’s orbit, Earth’s elsewhere in time, Earth’s elsewhere in the galaxy, and even what will happen to the Earth when the Milky Way and Andromeda galaxies eventually collide.
The nice thing about this for me is that he explores the science behind a variety of fantastical other Earths. In other words, he’s done much of the homework for exotic SF locales. Most of the science is well-written and aimed at the educated layman. A few bits got boring for me, but by and large it was good stuff.
However, this was not a particularly good Kindle edition. The text and diagrams themselves were decent, but the final 15% was taken up by a useless index (i.e. it had no links back into the text) and a collection of footnote/endnotes with no context back to the earlier text. Some of this may be simple limitations of the format, but I would have liked to have seen them handle it differently. If the index was going to be that useless, it should have been removed, and if there was no way to handle the footnotes more elegantly, they should have been inlined parenthetically in the text.
So, I enjoyed the book immensely, but I wish I had bought the dead-tree edition instead.