Turn to The Handy Space Answer Book, by Phillis Engelbert and Diane L. Dupuis. It's a sequel to similar volumes on weather and science. As in most such books, now and then the question-and-answer format seems forced, but it lends a conversational friendliness. The authors assume no prior knowledge of the subject, and hold forth enthusiastically. This is a wonderful introduction for anyone over the age of, say, 12.
Reviewed by Michael Sims.