It's been documented, but not at a large scale. Dave Burgess had his book Teach Like a Pirate owned (https://twitter.com/burgessdave/status/849039943561134080) and Bill Pollock says on this HN thread that five of his company's books were pirated.

If the experience of other manufacturers/brands are anything to go by (see https://news.ycombinator.com/item?id=13927641), I fear more and more pirates will get into books now that it's possible for them to own the buy box. It might not be a huge problem now, but I predict it's only a matter of time before it becomes one.