Dr. Alper's books were reccomended to me by a friend that attended one of my crystal seminars. She spoke of them very highly, and I have also found them to be wonderful reads. His books are essentially transcripts of channeling sessions, and they contain quite a bit of information on the uses of crystals. Some of the information pertaining to Atlantis seems to be very much akin to what we now attribute to Lemuria/Mu, although reading the messages still is useful and thought-provoking.
Dr. Alper's Exploring Atlantis trilogy should be read with a grain of salt, but is still highly appreciable.