If I may hijack your reply for a recommendation thread, I'd recommend "The Sense of Style" by Steven Pinker, too. It's worth it just for the section on metaconcepts. I loved how much simpler my writing became when I removed them.
Also, read anything by Robert Graves. He did write a style guide called "The Reader Over Your Shoulder", but that's optional; I learned far more about good writing from "I, Claudius" and "Good-Bye to All That".
Also, read anything by Robert Graves. He did write a style guide called "The Reader Over Your Shoulder", but that's optional; I learned far more about good writing from "I, Claudius" and "Good-Bye to All That".