> I also suspect that many of these self-help books are effectively a blog post stretched out over hundreds of pages, so maybe a decline in their sales is no bad thing.

You suspect right. I usually compare them to a pamphlet, but a blog post works too. They’re mostly one core idea padded with interminable stories to illustrate the point. Instead of wasting your money and time on the book, search YouTube for a talk the author has given about it (the more popular the book, the more of the same talk there will be) and watch that. It’ll hit all of the major points.