The purpose of this book is to help you create a sense of wonder in your speculative fiction by using real science. Gillett begins by delving into the astronomical, such as variables (like season change and tidal action) that affect planets, and some of the differences between stars and planets. He also helps the reader understand and calculate astronomical scales. He goes into the details of creating a planet, and here provides some variations, as well. He also takes a look at the Earth and some possible variations, and here he includes information on a planet's colors, the atmosphere, the ocean, plate tectonics, and more.
In addition, Gillett discusses Earth's earlier days, other planets, suns and stars, and more exotic possibilities for the planets you create. Also in the book are references and a resources guide.
An interesting book. If you want to write science fiction set on different planets, World-Building is a great way to get you started.
If you like this one, try: How to Write Science Fiction and Fantasy by Orson Scott Card, The Writer's Guide to Creating a Science Fiction Universe by George Ochoa and Jeffrey Osier, Space Travel: A Writer's Guide to the Science of Interplanetary and Interstellar Travel by Ben Bova with Anthony R. Lewis, and Aliens and Alien Societies by Stanley Schmidt.