I’ve seen this comparison before — and you probably have too — but here’s a link to another interesting piece that discusses the parallels between the rise of inexpensive, mostly-self-published eBooks and the rise of the “paperback original” in the early 1950s. I especially like that piece because it led me to a reprint of a short article by Bill Crider from 1971 (!) that gives a nice, quick little history of the paperback original. I probably never would have encountered that piece had it not been for the first article.