What is a reasonable estimate for the minimum length, in word count, for a non-fiction book? I'd guesstimate that non-fiction books "generally" (yes, this is very rough) run perhaps 150-450 pages, and at about 350 wd/page that works out to something like 50-150k words. (Correct me if I am off here).
However, occasionally very short books are published, such as Harry Frankfurt's On Bullshit, at 67 small pages with very wide margins (about 110 wds/page)...one count I saw was the whole book was only 3,000 words, as it was just a book version of a short essay. (It spent 27 weeks on the NY Times Bestseller list). This is obviously an extreme case.
But what of a book at perhaps 15-20k words? At, e.g., 300 wds/page that could be 50-67 pages. Aside from the Frankfurt book, is there any kind of market for something too long to be an article but too short to be a "normal" 250 pager? Other examples would be helpful. And what else should one know about that market?