Easily the best book on writing I've read. Closely followed by some research from the 60s on "process oriented writing" (I'm not sure if that's the accepted English term -- the material I read was in Danish). They essentially say the same thing: writing is re-writing -- and in that light, any tool that encourages you to look at what you've written again, and rethink it one more time, should help improve your writing.

[edit: Hemmingway himself has allegedly said "The first draft of anything is shit." -- but at least wikiquote has it as unsourced, so I'm not sure if the attribution is correct -- even if the idea probably is]