There are so many references it's difficult to cite only a few.
Here's a great magazine article by Pete Millett on the original Millett Hybrid:
http://diyforums.org/MiniMAX/history/ax_hybrid.pdf
It goes through many steps in the reasoning behind creating a simple tube-hybrid amplifier for headphones.
Another great link, although it's mostly about building, is Tangent's tutorial on building the CMoy:
http://tangentsoft.net/audio/cmoy/
Tangent has gobs of beginner information here, with the CMoy tutorial included:
http://tangentsoft.net/audio/
Like most design engineering, there is very little documentation or tutorials on the creative process itself. This is not really unique and exists throughout engineering. You will find many engineers are great at analysis, but it takes something special to actually create a design. Most get a start by reviewing numerous existing designs, learning how they work and building them, then making modifications and learning what changes what. Eventually, with enough background and preferences built up, one gets enough confidence to create a design on their own.
Your question is analogous to cooking and recipes. Many people can cook by following instructions and learning the basic techniques. However, actually creating a recipe for a dish that's tasteful ... well, that's another leap altogether.
I hope that's not too discouraging. Seems that some people have been accusing us of that lately.
