'The Guitar Amp Book' by Dave Hunter is very good for understanding how valve amps work. I've not read it all yet (hence the daft questions in Tech) but it is a very good book none-the-less. Other than that most kits will have instructions that take you through the build process (Guitar Amp Maker, Guitar Amp Builder etc). It really depends on whether you want to understand from a practical, methodical put the peices together side of things or the theoretical how valve amps work side of things. This book is the latter, albeit wothout the maths that would be required to actually design an amp. But it definitely gives a good insight to the thinking behind the design of many classic amps, plus some great interviews with famous designers.