Yes.
I have non-fiction titles ("How to" type books in a niche I know well and alternative health, which usually sells very well) and I've been selling since 2010. The sales on long-standing titles are down and sales are down overall. It looks about like the September "slump." My overall December this year was considerably worse (selling about 75%) than last year despite having added more titles. I don't do freebies and I write a lot of full-sized books, well-researched and documented. I, also, don't do much for promotion outside of writing some articles and having a couple of web sites and a blog. This means I'm not in the same category as people who write short stories or novels and put them in the Select program, who, also, seem to be having problems according to some threads. There's a thread here about this right now and it sounds like some people are having trouble even giving books away!
Just as I was in December, I am still concerned. I think the economy is going down in flames - I think we're at that point where things begin to accelerate right before the full disastrous impact. I'm not an economist or a doomsdayer; this is just how it feels to me.
When the economy gets bad, it's not just that people don't have extra money, they start having other problems (like losing their homes, dealing with spouses who are going crazy, a sense of total panic and anxiety, etc.) that put them out of the mood for buying and reading books.
According to some news articles, overall retail sales (of everything) were down over Christmas. Also, my lower sales aren't just at Amazon KDP, it's everywhere else I sell books, too.