I also could not produce a slap tone at all until I saw the drawing of the hand/finger positions for it in this book--again, because people cannot always explain things well, or different people tell you different things. This book is a great SUPPLEMENT to lessons and drum circles--I'd say you need both the book/CD and drumming with others to really learn and retain the information.
Oh, and the section on "How to Tune Your Djembe" is really a great thing to have.