I'd say the 5th edition Lizardmen Book goes a long, long way if it comes to fluff. In addition there's the Lustria Supplement of 6th edition. I don't think there are any novels. For websites, there is
http://www.lustria-online.com/ You might also check the Old Sage (
http://www.inisfail.com/oldsage/) A nice fansite with homebrew rules, fluff and even army lists.
It's information that is hard to come by, and it kind of suits them because they are very isolationist (and very successful at that!). Not many races deal with them on a regular basis. Do they even get a paragraph in the Big Book's world guide (if it has one)?
If I look at them in this way, I can barely understand why they are even a playable race. They're more Myth than part of the world*... and the "Myth that is Truth" part is already filled by the Skaven (their nemesis)... Does the Skaven Book mention the Lizardmen?
* yes, the Slann are the Old Ones' handyman race, but ehm...
GW once, quite sensibly decided that such did not justify a playable race... 5th edition brought them back as a quite slowed case of powercreep (Bloated frog-wizards that cannot walk unaided but can outfight a DRAGON? Seriously? Was Nigel Stillman a

or what?).