I was talking with an author last night and realized a change in the nature of published books the last few years. This author has published three books. These are self published books (now called independently published). Indie books are the fastest growing segment in publishing, made possible by Amazon and the rise of ebooks. In the past a new author had to sell a book to one of the large publishing houses. The alternative was to pay to have a "vanity press" print the books. Self published books were (often justifiably) assumed to be bad since publishers are supposed to pay authors, not the other way around. So prospective authors would often write a number of books before one was finally bought by a publisher.
Today authors can go the traditional route or they can go the new route of publishing electronically on Amazon. Alternatives exist but Amazon is the largest eBook platform around. Publishing a book on Amazon costs nothing, just sign up for an account, say where to deposit the royalties, and upload the book.
So back to the author I was talking with. He wants to write for one of the big publishers but doesn't think he's good enough. So he's writing multiple books a year in the hope he'll be good enough in the future for the big publishers.
After hearing this I realized that in the past an author's first, generally bad, work was discarded until the author is good to be published. Today authors are publishing these initial attempts, sometimes regardless of quality. If an author becomes a bestseller, people want to read everything the author has written. In the past the early work is mercifully unknown, or the author can rewrite it with the hindsight of experience. Today that early bad work is out there for everybody to see. I expect to see some number of embarrassed authors in the future.
This is not to say indie books are bad. Big publishing houses are known to publish bad authors, and a bestselling author may have to put out a "contractional obligation" book he never wanted to write. There are also a lot of very good indie authors. Some don't want to go with a large publisher. Others have a style or genre of book big publishers aren't interested in at the moment. Established authors sell their older work as ebooks since the big publishers don't keep books in print more than a couple years anymore. So there's a lot good stuff out there but also a number of practice books.