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.
Lukeprog has done some great posts talking about how scholarship has benefited him personally and how to do it. However, he hasn't talked much about how to tell the good from the bad, and I think this is an especially big problem with it comes to "how to"s, rather than standard academic subjects.
The thing is this: writing on academic subjects tends to be dominated by academics seeking to gain status from their work. Not all fields have a strong correlation between status and making actual discoveries, but many (perhaps most) do. So for academic subjects, the big challenge when it comes to making sure you're not reading nonsense is to try to evaluate the soundness of the field in question.
When it comes to seeking out "how to" advice, though, writing on the subjects seems to be dominated by people looking to make money selling advice. That means that if bad advice will sell better than good advice, they'll give the bad advice. Even if their writing includes legitimate advice, they have incentives to give a distorted picture of the subject they're writing on if it will sell better (for example, making whatever they're talking about sound easier than it really is). In some cases, advice books end up trying to nudge you towards buying some more expensive thing the author is selling.
So: when you're researching a "how to" topic, how do you tell the good advice from the bad advice?