One of my favorite books of all time, Apollo: The Race to the Moon by Charles Murray and Catherine Cox, went out of print soon after it was published in 1989. A friend of mine happened to own a copy, but when I went to buy one for myself, only expensive collectors’ editions could be had. Recently the book was finally picked up again by South Mountain Books. The publishers of South Mountain Books were… Charles Murray and Catherine Cox. This book had rave reviews and a core of dedicated fans, but because of its position down the long tail of all books, no publisher would take it until the authors themselves took on the job. Go buy it now.
Most happily, publishing your own book isn’t so hard anymore. Murray and Cox may have a few more resources at their disposal, but you can use Lulu.com. It’s hard to beat their value proposition: just-in-time vanity press. No need for expensive set ups and big speculative print runs. Lulu lets you publish and sell your book on a print-only-as-needed basis. Charge whatever you want; as long as you cover your per-book costs, the rest is yours to keep.
This kind of publication will change the world as surely as Gutenberg did. Even so, it’s only half of the print-on-demand problem. Lulu addresses books that have never existed before. We still need good solutions for the Apollos out there, the orphaned books that have gone out of print.
Rambles Blog at Star Chamber 
I actually have some friends who work at a branch of Lulu out in Raleigh. It doesn’t seem like too bad a gig.
In fact, we published the book ourselves just because we wanted to do it that way. There was more than one publisher who wanted us to bring out a new edition. Our goal was to make, and keep, this title available to people interested in space and in the history of the Apollo program. There were amazing people involved whose stories aren’t told elsewhere. I mention this because it’s relevant to the points made above about self publishing.
Thanks for the note Catherine!
I stand corrected, and this new information seems to underscore the value of self-publishing. Most of us would need to self-publish in order to be published at all. But even people being actively recruited by publishers, people who have other options, are choosing to self-publish.
What do publishers provide? Cash advances, editors, distribution, and marketing. If you don’t need the money upfront, then you’re in a good position to take care of the rest by yourself.