I believe the whitepapers are still supposed to be available in the customer portal. I seem to remember them popping up on the forum once in a while, but I did not find them with a search just now.
Most of the callbacks are not documented. There are whitepapers available for the hub, controller, and NAS, and those whitepapers contain descriptions of all of the callbacks for those components. Most other probes have callbacks as well, but they do not have whitepapers. Your best bet is to explore the callbacks with the Probe Utility (Ctrl+P while the probe is selected). You may have to be a bit careful not to invoke a callback that would cause you trouble, but most are not much trouble.
And if you have some questions about what something does or how to find a callback that does something specific, you can find a lot of people on this forum who have done played with a lot of them.
-Keith