PeterM,
I have not experienced the same thing, but this may be due to several reasons:
a) either there aren't many books on the niches I am targeting as of yet.
2) I typically go for very specific keywords within a niche, so maybe those books are triggered to show up in the search results for generic/general keywords. In the example you mentioned, what kind of keyword was it? General or very specific?
Certainly, the web changes over time, but I really don't think that the niche sites will be phasing out of the search results any time soon, if ever... but who knows
