It's a real shame that "LITHOPS - Treasures of the Veld" by Steven A. Hammer is out of print nowadays, because I'd buy it right away. But at least there are .pdf files floating around the internet so that we are able to enjoy this great book digitally. I've had it for a while, but only just read it. It's truly amazing. Not only very informative - it answers so many questions I had on my mind! - but also a fun read. I had to laugh a lot while reading it. :) You can feel how dear these plants are to the author, he notices all those little things about them and writes with such passion - reading it was a delight. I'm sure I will re-read it often enough in the future. (How wonderful it would be to have it on the bookshelf and not on the computer. They need to re-print it, seriously.)
Both Mr. Shimada and Mr. Hammer refer to "Lithops: Flowering Stones" by Desmond and Naureen Cole as a standard work on lithops. I'll have to put my hands on this one, too. So far the former has the tons of pictures, the latter - tons of information. Desmond Cole probably has both. :)
PS: if you want a .pdf of this book feel free to drop me a line in the comments.