I enjoy some self-help books because they seem to articulate an underlying gameplan for pursuing whatever one considers "success," which are ideas my parents and school only taught me implicitly or inaccurately. My parents' advice to me as a kid was "do what you love and the money will follow," which is not true, and/or "join the Navy," which I had no desire to do. So I've ended up trying to find my own way in life, however imperfectly.

Essentially, a lot of these books are about executive functioning skills, the meta-level planning and goal-setting and failure-analysis efforts that are necessary to make progress in whatever field. I think it is also true, as other commenters note, that to some extent they fill an ethical or existential void left by the decline of religion.