Mountain Flying & High-Performance Resources?

I'm looking to find more information related to earning my
high-performance endorsement. Primarily, I'm wanting to see if there is
a good, clear, concise, simple website or book that explains the
transition (in thinking and practice) from a plane that uses only a
throttle and mixture control to a plane with throttle-prop-mixture and a
Manifold Pressure gauge. When I go up with an instructor to start on my
high-performance endorsement, I want to have some understanding of what
I'm doing, when I'm doing it, and, most importantly, WHY I'm doing it.

Also, What is a good book out there for learning about mountain flying?
I seem to remember that Imeson's book's are pretty good, but I don't
have any solid knowledge of what books are better than others. I do
know to stay clear of the cumulogranite clouds often found in the
mountains. ;-)

Thanks,

Chris G.
PP-ASEL
Salem, Oregon