I was sitting in Starbucks reading it last week and a middle-aged man commented: "Best book in the world." Turns out he is a teacher who focuses on creative writing; he reads Shrunk & White once a year.
It's short: 105 pages including the index.
It's practical: 11 elementary principles of usage; 11 elementary principles of composition; 21 pointers on style; and a great chapter on "Words and Expressions Commonly Misused."
And it's already reduced my word-count! Amazing how many superfluous words we use in writing!
Give it a try.