Andries,
Your book idea is very good. The idea could be pitched to a literary agent specializing in this field. I wrote "Unbounded Light" in 1992, and used a well-known New-Age agent. It was published by Samuel Weiser and sold about 10,000 copies. I have corresponded recently with my agent. He has recently read "Enlightenment Blues," and he corresponded with that author, who I believe is Dutch.
The marketing for another book on cults might not have a real high potential at this time. I'm not really sure. I considered doing a book all on my own, in a format exactly as you describe, but I have another publishing project going that is taking up all my time and is a lot more positive than a book on fallen gurus would be.
On the other hand, there is a plethora of first-hand accounts from ex-devotees that make for interesting reading and these could probably be gathered into one book fairly easily, without having to pay too high permission-to-reprint costs. The book would certainly serve a purpose.
Initially, since many first-hand accounts already exist on the Internet, instead of a published book, a new Internet site could be set up where a highly representative selection of different cults could be collected. A site like this would provide a valuable lesson for many people, mainly it would show just how typical all these gurus are and how their con is so similar.
If you started such a site, then I would be happy to help you collect and choose the best stories from each of the living gurus, such as Kriyananda, Gurumai, Cohen, Sai Baba, Da Love Ananda, Eckenkar, Elizabeth Clair Prophet, Sun Yung Moon, Ganga Ji, and yes even Ammachi, and a long list of others too numerous to mention. Once the site is complete, a publisher might want to put in on the market in print form.