Standard disclaimer, I am not a lawyer. Having said that, I think there is a difference between reworking an idea that is pretty well known in the subject area (that is, if you pick up five books on astrology and four of them will at least mention the idea) versus reworking something that one particular author discussed that might be their innovation. If it is a generally known concept that you are explaining better, you are probably fine. If you are explaining that one author's special theory, I think you have to be a lot more careful. Give credit where credit is due, something like "as so-and-so explained in the-book-name..." With references, you won't run as much of a risk of unintentionally plagiarizing and may be sending some additional sales their way.
Also, I'm hoping that you are not only re-working material from elsewhere, but including your own innovations and insight -- value added.