From Martin Weller:
Now ask yourself, how many academic books (or even fiction) have you read that were really a 40K word idea stretched out over twice that length? Me, I'd say nearly all of them. This is a classic example of old conventions dictating the possibilities of the new. My book will be available freely under a CC licence as an epub and PDF version. There will be a physical copy available at a reasonable price, so the need to make the book 80K words in length diminishes. I had made the case I wanted to make, explored it in depth, and kept it reasonably concise. People might even read it.This isn’t a new thought. Back in 2009, Philip Greenspun wrote: "Suppose that an idea merited 20 pages, no more and no less? A handful of long-copy magazines, such as the old New Yorker would print 20-page essays, but an author who wished his or her work to be distributed would generally be forced to cut it down to a meaningless 5-page magazine piece or add 180 pages of filler until it reached the minimum size to fit into the book distribution system. "
That said, Kindle Singles and long blog posts notwithstanding, I’m not sure that mainstream publishing has changed all that much. The gravitas that a book brings still requires a certain thunk factor as we used to say when writing reports when I was in the industry analyst biz.