Hm, I think "self help" might be unfeminist in its approach in general.
I can only fuzzily remember the self help books I have read, but they seem to be an awful lot about positive thinking and bootstraps.
Feminism is all about recognizing how the personal is political, and you and your problems don't exist in a vacuum.
What problems do you want to solve?
There are books that recommend women to train men like dogs to achieve happiness in relationships, books that tell women how to imitate men in the workplace to achieve success ... the problems are obvious.
So yes, I am very sure that some self help books exploit vulnerability. I think there are areas where they can be useful, but there are other areas, where they distract from the need for political activism.
I would stay away from books that are explicitly targeted at women but not political in a feminist sense, and moreover only buy a self help book if I am sure (without having read any self help advice) that I am the main cause of the problem, and if the book doesn't just contain encouraging words, but actual strategies for change.
For example, I would buy a book on keeping one's living space in order, as the untidy state of my space is not caused by anything else but me. (I have the time, I just don't use it)