http://www.chromium.org/spdy - contains some great and in-depth research motivating SPDY.

Original whitepaper: http://www.chromium.org/spdy/spdy-whitepaper

Here's a good tech talk to checkout as well: http://www.youtube.com/watch?v=TNBkxA313kk

Draft 3 has also updated the dictionary, here's the research paper motivating the change: http://www.eecis.udel.edu/~amer/PEL/poc/pdf/SPDY-Fan.pdf