I just wanted to bring this post that Lavenderfish posted since it is a recommended book.
Bluesssman, Figured this thread was already going so here's something more.....
Check out
http://www.harmons.net/ I have the 2 books at the bottom of the homepage and would highly recommend them. They're full of background info on the agates history plus knock your sox off cabs, carvings, display slabs, etc. Rockhound eye candy galore!
The carving book gives good descriptions/advice on how to "read" the nodules to decide which way to orient them for cutting. Also detailed description of how the author (Tom Harmon if I remember right) does his carvings complete with what tools, grits etc that he uses. Lots of photos
too.
Worth every penny!