Here an address, introductory to the field. I find it very interesting.
A book that I think deserves bought, if passion in the field increases. It's not cheap, but it seems to be kind of hard to find book. Maybe he could be found at any library.
Once known names such plants can, you look at special nursery, searching on net.
Or you can ask the collectors associations of succulent plants.
Hetty, if you find something, let us to know. Really interesting, I did not read nothing yet about this plants.