Tung has posted some really outstanding links here, just a treasure trove of amp and electronics information. With the intention of building my own 18W Marshall clone in early '08, I asked him for some good reference materials and, man, did he come through!! If you think you might want to try amp-building at some point, or if you just want to understand better what exactly is going on inside that tolex-covered box your guitar plugs into, check some of these out.
Thanks again, Tung!! : ude: