I know a guy who self-published a couple of how-to books on using Windows Movie Maker and just sold them through Amazon and his website. He figured that there were only a small handful of sales in a book like that anyway, so no traditional publisher would probably want it, and even if they took it on, the book would sell in such low numbers that his end of the profits would be next to nothing. For him, self-publishing was probably a good idea. I can think of a number of situations like that, especially with how-to books of limited interest.
The same would likely be true of a lot of scholarly titles too. For instance, Oxford Press regularly comes out with books that sell less than five hundred copies total. I had one from them that was basically just a bibliography of the works of a 19th century Scottish author who was quite good, but not well-known. I think only 200 of those books were ever printed. The author probably didn't make $25 on the deal. Self-publishing has its place.
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