What If the Earth Had Two Moons?: And Nine Other Thought-Provoking Speculations on the Solar System - Neil F. Comins First the "good stuff":

The ideas alone get a minimum three+ stars in my reckoning. I've always liked SF stories that took me to truly weird worlds like Mesklin (Mission of Gravity) or Helliconia (Helliconia: The Classic Epic Trilogy in One Volume) and how the environments shaped the natives and the humans who (sometimes) colonized them. So in this book we get 10 scenarios about what an Earth-Moon system would be like if something were changed:

1. What if the Earth had two moons?
2. What if the Earth were a moon?
3. What if the Moon orbited backwards?
4. What if the Earth's crust were thicker?
5. What if the Earth formed 15 billion years from now?
6. What if there were a counter-Earth (ala Tarnsman of Gor, et al.)
7. What if the Earth had formed elsewhere in the galaxy?
8. What if the Sun were less massive?
9. What if the Earth had two suns?
10. What if another galaxy collided with ours? (This last scenario is going to happen in another 3 billion years or so, so if we have any descendants around at that time, they'll get to find out firsthand.)

Comins writes clearly, and even the most astronomically innocent reader should be able to grasp what's going on. The problem is the execution. Comins manages to make what should be some really exciting ideas sound...well...dull. Pedestrian. Ho-hum. And it doesn't help that he employs two extraordinarily annoying tactics. One, he introduces each chapter with a (mercifully) brief fictional episode set in the alternate Earth we're about to discuss. And, two, he feels he has to rename his Earths, as if you couldn't keep our Earth separate from his speculations. I skipped the fictions and gritted my teeth and tolerated the naming conventions.

I'd give this book a qualified recommendation to astronomy fans and SF authors looking to find exotic scenarios to put their heroes in but overall I was disappointed.