as some comedian has remarked, the idea of getting self-help from a book is already something of a contradiction

I agree but not sure if for the same reasons. I think most of the time people know perfectly well what they should do differently, they just lack the willpower or motivation for it. A book here may inspire for a short while, if it is really well worded it can "pump" people for a while, but it will not last long. In the vast majority of the cases, people buy self-help book, read actual good advice in (generic good, not big insights, mostly the "get your sh.t together" type of good), nod, nod, and then do nothing.

Coaches, trainers probably work better. So do groups. I think the core idea of AA is that every meeting gives a jolt of motivation enough to last until the next meeting.