On Writing Well is one of two American classics on writing. (Also Elements of Style.) I can't count the times I've read it straight through. I can't count the times I've referred to it.
If you're a writer and don't own On Writing Well, go get it and read it. Read it again, too. You're skills will improve exponentially. You might want to visit his publisher: HarperCollins
And, oh, by the way—did I tell you I have some cherished personal, handwritten correspondence from Mr. Zinsser himself? You can read all about it right here.