Well, obviously this is the hardest part of designing a shader.. You start with an idea for an effect usually, no matter how crazy it sounds, and then you should check the internet to see if you aren't re-inventing the wheel as for the method for the shader.... I know some guys who produced some very nice effects this way, but most of them are professionals and they won't even start explaining me (I don't blame them)...
I tend to fiddle a lot with programs like FX Composer, RenderMonkey etc. etc. just to look what changes what.. And off course read a lot about it ; though believe me I'm far from a shader pro....
Anyways as for shader making trully from scratch you've got to know all the ins and outs of 'everything' I guess, for example if you plan on making the multi-terrain shader instead of alpha-based, height-based, you'll have to either know or figure out how to get/calculate the heights of a vector/pixel and write some code which makes sure that (for example) the icy texture is only there on the terrain parts above a certain height... Off course before you can even start to think about shaders in this way, you should know something about socalled 'passes', the syntax (functions;what describes for example 'height', so the actual basic setup for a shader. It seems with the two books you've got, you should be able to figure most of that out.. When you have an idea, try to somehow debate with others about it, that happens a lot at these forums, and they will give their own ideas about it, maybe giving you something to start with....? I mean unless you start asking; "I want this shader please?!! NOW because I need it so bad and...." etc. everyone here would share their ideas, I'm pretty sure...

Cheers