I've been selling them for 3 or 4 years now an have collected a decent amount of information on them. As you figured out, there is not much on the net about them.
The best book out there so far is one that was only published a month or so ago. Zoo book sales should have it in a couple of weeks if they don't have it already.
The book is Giant Tortoises of the Indian Ocean, Justin Gerlach, 1994
I don't agree with the taxonomy but everything else is fantastic. You will not find as much information in one place anywhere else.
Zoo Book Sales
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Ed
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Trying to keep the fun in Chelonian care