Without reading their ebook, I don't really know enough to say whether or not it works. Even if the subconscious mind can absorb information while you sleep, how practical is it? Mastering any subject requires far more information than would fit on any tape. Of course, you could make several tapes, but that would be very time-consuming and that time would probably be put to better use by studying. Since the brain is in theta for only a few minutes at a time(during sleep), you would have to know exactly when your brain would be in theta ahead of time. It seems like the amount of information you could absorb at any one time would be extremely limited.
Again, without actually reading their book, I can't really say for sure. There may be something to it. There have been times that I have fallen asleep in front of the TV before and I dreamed about the show I that was on while I was sleeping. The subconcious mind must be fairly active during certain sleep cycles. But if sleep learning is such an effective way to learn things, then why isn't there more of a market for products like this? It just doesn't sound practical to me.
