I've read Harry Bauld's book, 50 Successful Harvard Application Essays, Fiske's Real College Essay's, Collegeboard's The College Application Essay, and Essays That Worked.
I liked Harry Bauld's book, but I expected it to be better than it was after it was mentioned on this forum serveral times. It was more directed to parents and it's very thin and expensive. Collegeboard's The College Application Essay, is alright. There are a lot of SAT plugins and shameless marketing, but I think the brainstorming ideas are a lot better than suggested in Harry Bauld's book and it helpmed me pick my essay topic. Overall though, it's not worth buying, even though there are some good essays in the back. Essays That Worked was my least favorite, probably because I checked it out from the library and is one of the older additions so the essays arn't good. At all. Fiske's Real College Essays has good examples, but no critique of the essays. 50 Successful Harvard Application Essays is similiar to Fiske in that it provides good essays, but also analysis of the essays, which I found quite helpful. However, The essays sound TOO good and most high school seniors (including myself) don't have such an ability to write. It is fun though to just read the essays in this book. I havn't read Princeton Review's new book on college essays, or 100 Successful College essays yet, which I heard is quite good.
Overall, In my opinion, I don't think college essay books are necessary. After reading a handful of essay books, they basically all say the same thing (Harry Bauld just says it the best). It might be helpful to get a book with application essays in it (Which I would suggest 50 Successful Harvard Application Essays) to get inspiration to write your own, but other than that you are writing a topic about yourself and you don't need a silly book to tell you that.