I'd like to be able to see what registry keys are used for. A book on the registry would be fine as well. Info on Windows Vista changes would be great!
Oh, how I wish there was a single site where the registry was collectively documented by all who write values there! Keep in mind, any app can create its own set of registry keys and values, so it's not just MS who would be necessary to such an effort. Though they would be a key player!
In the meantime, there are two MS sites I use as startng points when I need to know what a particular key/value pair do:
Windows 2003 registry reference: http://technet.microsoft.com/en-us/library/cc778196.aspx
Windows 2000 Registry reference: http://technet.microsoft.com/en-us/library/cc974061.aspx
Keep in mind that since Windows is an evolving thing, even information about older versions can be very helpful. Also note that W2003 bears a lot of resemblance to XP. Sadly, I have not found a link to the Vista/Windows 2008 registries similar to the above.
When you need to know what's up with a specific key, and it's not found in one of the above links, try a search of that key's full path at http://support.microsoft.com