This book covers just about everything you might want to know about creating aliens and alien societies. It starts with a couple of basic questions (what are aliens, and why would you want to write about them?), then moves on to achieving plausibility in your writing. Astronomical and biochemical essentials are explored, as well as the importance of making your aliens fit their environments, creating alien cultures and languages, and how aliens might interact with humans.
Schmidt also goes into alien motivations, and gives some samples for the reader to study. He closes the book with a discussion of more exotic possibilities before giving an extensive list of references and a glossary. Schmidt peppers the text with examples from both his own and others' writing, and whets the reader's interest for more. If you want to write about aliens, you'll want this on your bookshelf.
If you like this one, try: How to Write Science Fiction and Fantasy by Orson Scott Card, World-Building: A Writer's Guide to Constructing Star Systems and Life-Supporting Planets by Steve Gillett, The Writer's Guide to Creating a Science Fiction Universe by George Ochoa and Jeffrey Osier, and Space Travel: A Writer's Guide to the Science of Interplanetary and Interstellar Travel by Ben Bova with Anthony R. Lewis.