I think there are a couple of takeaways from looking at the books that pile up huge numbers of citations in philosophy over the last twenty years:

1.  Books that draw an audience outside philosophy fare much better–in part because citation practices are stingy in philosophy but profligate in some other fields (e.g., psychology).  This makes the high citation counts for books like Williamson's and Hawthorne's (books that only analytic philosophers are going to read and discuss) notable.

2.  Gender (and somewhat recently) race have become "hot" topics in the humanities and social sciences, so philosophy books that touch on those also fare well.

3.  Some of the books are actually quite good and worth reading!