Self-publishing, combined with print-on-demand, is something of a distruptive technology. It removes a huge barrier to publication, namely convincing a publishing house to take on your manuscript. Even if they do, for a book with projected low sales (for example, a book on cheminformatics), this usually means high prices and fewer readers.
With self-publishing, all you need is a PDF (or even Word doc), upload it over at Lulu (or one of the alternative self-publishing websites), and a couple of questions later your book is available for sale. No fee. And it's profit from the word go - a proportion of the cost of the book pays Lulu, and the rest goes into your pocket. Getting it listed on Amazon costs a few pounds, but it's also very easy to do.
I first heard about this in cheminformatics when Angel Herraez published the Jmol handbook (also available in Spanish) back in 2007. He makes the interesting comment on the book website that it's fairly easy for him to generate a customised version of the book for use on a course - I don't think that'd work with traditional publishing.
More recently, i.e. yesterday, Egon Willighagen published Groovy Cheminformatics with the Chemistry Development Kit. This covers the use of the CDK cheminformatics toolkit. The Groovy in the title is not the 70s vibe, but rather the Java scripting language.
Makes you think eh? What would you write a book about?
Update 13/02/11: The Cactvs Scripting Manual is also self-published with Lulu (see comment by WDI below).