I have that (Strunk-meister) but never really consult it. Stephen King's (let's put aside what we think of his works for the moment) On Writing is a great style guide even though it concerns writing short stories and novels. Read the technical (non-biographical) part of that book a couple of times and you'll see the words, sentences, and paragraphs that you're cranking out a whole lot differently, I guarantee you. Plus, it's an easy read.
I find his fiction enjoyable; it reads easily and his sense of humor is similar to my own. (It is possible to overdose on his style, as I did on Vonnegut after a few volumes.)
"Single greatest book on writing of all time?" We'll part company here too. I read On Writing shortly after it came out, and was entertained through the front half, but can't for the life of me remember a shred of advice from it. I do recall the striking discontinuity in the last part, where he writes about the auto accident that nearly killed him. The writing was obviously different from that in the front of the book, and I felt the topic was out of place, and even self-indulgent. But he gets a bye, 'cause he's Stephen King.
If you are starting off writing, and you're not a freak of nature like David Foster Wallace or James Joyce, and you can get over any revulsions you have about Mr. King, and you'd like to be shown the tools of the craft by a friendly craftsman, for what it is worth I can recommend it. And I want to say thank you (not) to that girl who borrowed my copy and promised to call me and return it and didn't - you suck just a little bit.
