Do you happen to know whether the DOJ's figure for number of books published included self-published books? Some of the dreck on Amazon is likely to have only a dozen readers.

There are a couple of "all you can eat" sources which do in fact pay royalties for back catalog books. How much I don't know. One is Kindle Unlimited which has, besides the dreck, some actual once-bestsellers and back catalog from people you have actually heard of.

The other is Everand, a spin off from Scribd. For about $90/year you can get really a bazillion back catalog books, though not every author goes with their program. For SF fans, if you are into Poul Anderson there are 141 books available, thought their search engine picks up books by others that Anderson may have done a blurb or introduction for and some are translations into other languages. . For Clifford Simak, 545 (same caveat--for SF short story writers some of the hits are for copies of the magazines they published stories in. ). For the mystery fan Donna Leon has at least 29 titles, including some of her non fiction. Stephen King has about 90, though I didn't bother to check for duplicates.

Both these sources are ebooks. It's hard to imagine an "all you can eat" place beyond a large library that could distribute DTBs.

Everand also has audio books of quite recent and popular titles.

Back in the day the original Scribd site had copyright issues because folks could upload things that violated the copyrights. Everand doesn't let people upload so that problem is avoided.