(My book will be considerably thinner, and less influential, than this)
I sent the manuscript for my Battle for Open book off to the publisher Ubiquity Press last Friday. I can't find the origin of the phrase "a book isn't finished, it's abandoned", but I was contemplating it last week, in trying to decide if the book was finished (or at least bar copyediting and review feedback). It came in at about 57,000 words. That's quite short for a book, and my initial reaction was "create another chapter or two to bring it up to 70,000 plus words". Having written three books previously, they've all been around 80K words.
But then I caught myself and thought "why?" Convention dictates that books are around that length, but that convention is really the result of the economics of book publishing. It doesn't cost much more to print and distribute a 50K, 80K or 100K word book. So they are priced the same, but people tend to shy away from purchasing thinner books. The convention then settles on 80-100K words being the optimum.
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.
How long should a book be? As long as it needs to be. Won't that be refreshing.