My two favorites:
1. Overcoming Autism: Finding the Answers, Strategies, and Hope That Can Transform a Child's Life. Lynn Kern Koegel and Claire LaZebnik. This book combines an expert's recommendations and a mother's comments to make a very readable, smart book.
2. Helping Children with Autism Learn: Treatment Approaches for Parents and Professionals. Bryna Siegel. Dr. Siegel's approach to education recognizes that each child with autism has unique strengths and deficits, and she offers strategies for using the strengths to minimize or work around the weaknesses. Practical, useful, and packed with information, Helping Children with Autism Learn is invaluable to parents and teachers of children on the spectrum.
Another book that isn't really about autism but is on my list of top five nonfiction books EVER is Temple Grandin's Animals in Translation. I couldn't put this book down and plan on rereading it soon. Grandin has autism, which she feels gives her a unique insight into animal behavior. A blurb on the back cover, from Nature, says, "At times, it is difficult to work out whether this is a book about animal behaviour with insight from autism, or a book about autism that uses animal behavior to explain what it is like to be autistic. A major achievement of the book is that it is both." I totally agree.