This book shows concise, straightforward guidelines to flesh out detailed yet usable sci-fi concepts.
Several designs spacing from aliens to starships are broken down in a step-by-step fashion, from the blue pencil sketch to the finished piece in either traditional (markers and ink) or digital (Photoshop); that said, the book is always valid regardless of what medium you're using. Each step brings some "happy accidents" to the table, and the author's method relies on building on top of these unplanned elements to flesh out a more interesting design.
Mr. Chiang assumes the reader is a good draftsman already which means the book is primarily focused on method. There's also general guidelines on design (strong silhouettes, resting areas vs. detail, leading the viewer's eye).
Overall it's a great book for artists who need to develop sci-fi concepts but don't know how to approach the genre. The quality of the artwork is great and the book is pleasant to read. I wish the book were more detailed on the actual subjects depicted (e.g. joints and other mechanical parts, equipment, etc.), but it's nothing that looking for your own reference doesn't cover. It's definitely worth its price.