I'm sure that whoever wrote this topic didn't mean self help books. And typically, self help books are not my thing...and I don't particularly find them all that helpful. But...
The book that has had the biggest impact in my daily life is called Fighting For Your Marriage, by Howard J. Markman, Scott M. Stanley, and Susan L. Blumberg. I know, I know, I'm not married. And although this book is geared toward those having issues in their marriage, the information in this book can be used anywhere. Basically it teaches you how to communicate your thoughts better, and understand others better. It teaches it within a relationship, but you can use it anywhere.
I read it about 10 years ago, and once in a while I pick it up for a review. But I use it everywhere...at work, home, with friends, family, everyone I meet! And since I've been using the concepts in this book, its been much easier to communicate with people and understand what their saying. I have a lot less miscommunication in my life now then I had before, and that means less conflict with those around me.
I know this is probably not what this question was suppose to be, but this is my honest answer. Its not asking the book I like the most (I could never pick just one), or have read the most times (The Whims of Creation, by Simon Hawke), or my favorite series (Exiles Series, by Melanie Rawn). It just wants to know which one has an impact on my life, and as boring as it might be, this is it!