In my opinion they're an obscure group of plants that most traditional Cacti & Succulent collector's growers don't have too great an interest in. Probably a little too "fringe" for most of us, myself included. Plants are not readily available, a lot aren't overly attractive to begin with and many can be difficult to grow in culture.
I admire Attila's efforts, and bought his first book, but not overly sure that a second book is warranted - at least as far as the hobbyist is concerned ??