A recent study has found that AI-generated books now account for 20% of Amazon's self-published catalog, but only generate 12% of sales. Meanwhile, revenue per book for human-written titles is declining in seven out of eight genres. This shift in the market could have implications for authors and copyright holders. The data may also play a role in ongoing lawsuits against companies developing AI-generated content. This development matters as it highlights the growing presence of AI-generated content in the book market and its potential impact on human authors and publishers.