There is no monolithic entity called "the publishing industry." It is instead a chaotic accretion of niche markets and firms. Even the Big Five act more like accretions than like integrated wholes.
This matters because there is no valid way to measure "the publishing industry" as a whole. As a specific example, fifteen years ago nobody would have put Tyndale House down as a major player in the publishing industry; the commercial success of the
Left Behind series and its offshoots, spinoffs, etc. has changed the publishing industry forever (or for five years, anyway, which
is forever in publishing). Similarly, fifteen years ago there was no Amazon, and nonbookstore sales of trade books were considered insignificant. And so on.
As far as the "fiction/nonfiction" breakdown, it has historically (that is, since the end of WW2, when aggregate figures became comparable from year to year) been about 75 nonfiction/25 fiction in trade publishing, with some variance but no real trend. Keep in mind that the big players in trade publishing are pretty quick to pick up on fads from other publishers and publishing segments, so trends of less than five to seven years of constant, predictable direction and magnitude are probably meaningless… and nothing I've seen indicates any such trend in progress at this time.
Whether Americans are "still buying books at the same rate" depends on both what you mean by "books" and what your baseline is, let alone whether you mean in the aggregate or per capita. Then there's the whole market-segmentation issue I alluded to at the top.
And thus, the short answer to this question is the same one you'll get from just about any well-trained lawyer on most questions:
It depends.
