The jury is still out on serial ebooks, but John Scalzi is giving it a good try right now. And Baen have been selling ebooks on a not-dissimilar basis for the past decade – you can buy up to the first half of an ebook three or more months before the publication date. Meanwhile, I’d like to note that this is how most novels were sold in the early 19th century. The current paradigm of the monolithic finished text being delivered in one installment is the new interloper business model, if anything.