I agree with the majority of gmatclubers who recommended Montauk and Bodine. Although I like both books, I've found Montauk slightly more usefull.
The third book I've bought alongside these two is "65 successfull Harvard Business School essays". I think it is a great book and everyone (not just HBS applicants, my example - I'm not) shoud have one.
The book, as its name implies, contains 65 essays that are divided into several categories based on HBS essay questions from recent years. Most essays are written to meet 500 word limit, so they are short, concise and simple, which busts the myth that one has to be Shakespeare in order to get into business school. Explanations that are provided at the beginning of each chapter are usualy 1-1.5 page long, but are so well written that I was amazed. In those explanations the author states what are necessary parts of a succesfull essay and what should be avoided. In addition, in the end of each essay is given a short explanation (usualy 1/3 or 1/2 of the page) in which author explaines why that essay was successful and how the candidate had included all necessary elements.
To conclude, very usefull book, to be honest, I've been using that book much more than the Bodines' one. My advice - buy all three books, you'll prefer one of them, but which one - depends on your style and your needs. But you can't miss with any of these books - they are all excellent.
My GMAT experience
My retake experience - GMAT dropped from 720 to 690 - advice for retakers
My TOEFL experience - 108