Well, it’s kind of like building a do-it-yourself doghouse based on instructions from the book.
Maybe you know how to do most of the things that go into building a doghouse. Maybe you’ve already even built a doghouse or two, but were never really happy with the results.
If the book on doghouse-building addresses the areas you are weak on and overall is well-written, authoritative, and informative, you might end up with a really great doghouse, which you did build yourself—but you needed some advice from the book on how to do it. Most self-help books (the ones that actually work) are more like that than you think; there is work for you to do. If you just read the book, the book does not help you. Only you can help you.
To be honest, the only self-help book I ever really used was one on how to improve your handwriting, and it was really great. It was my many hours of practice that improved my handwriting, but I couldn’t have done it without the book’s advice and examples.
Also, I do http://www.flylady.net which is basically an ongoing self-help book, and that’s working pretty well for me. What I learned from that is that sometimes you can think that you already “know” something, but still need someone to repeat it (more times than I’d like to admit) before it really sinks in and you see how to apply it in your life.