I think Tut1 has a lot of good info for a designer. It covers the basics in the preamp and power amp pretty well. The sections on modding Fender preamps is also a great source. The channel switching is also good.
My only complaint (and it is mild) is that O'Connor's books seem to discount anything he doesn't like, such as EL34s or using optoisolators for switching, both of which are more than valid choices. I prefer to overlook the opinions and use them as good references. All of the Tut books seem to center on a few topics and complement each other.
At the end of the day no one book or series of books is enough. You'll need a small library. I think the Torres book "Inside Tubes Amps" is a good beginner book. Even the Gerald W.e.b.e.r books have a lot of useful info (even if they are more maintenance and common mod oriented).
As a designer, I find getting ahold of schematics of amps you like the sound of and want to emulate is also very valuable. Once you have a little theory, you can see why a Vox is different from a Marshall or Fender and how to use those differences to arrive at sounds you like.
I know
http://www.tubesandmore.com has a nice selection of builder oriented books.