Probably more accurately the “five types of books looking for publication“, i.e. things like “books with unknown markets” rather than “books that are unpublishable shit”.
Note that I disagree with Baldur Bjarnason’s final argument here, which is “go big or self-publish”, i.e. go with tradpub if you’ve got a massive blockbuster, otherwise do it yourself. Big publishers still buy and, in my opinion (and experience), still bring a lot of value to small and mid-sized projects, so long as authors are cognisant of what they’re getting into before they get into it.
That being said, I do think he’s right that, for a lot of projects, selfpub is the way to go. There are a lot of books out there that aren’t commercially viable from a tradpub (a.k.a. commercial publishing… compub?) point-of-view, but that still deserve publication. These are the projects selfpub was made for. Unfortunately, as selfpub becomes more and more (and more viable), I think the number of projects that will fall into the “default selfpub” basket is going to get bigger and bigger, while the number–or, more accurately, the number of types–of project attractive to commercial/traditional publishing is going to get smaller and smaller. “Compub”, in other words, I think will find a “niche”, in the same way Hollywood and the AAA gaming studios have a “niche”; that is, producing massive blockbusters along a very narrow set of profitability guidelines (“military shooter”, “superhero film”, “MOBA”, “slightly misogynistic thriller”, etc.).
For everything else? I sure hope you’ve got the money, skills, and time to DIY.