In Finland 30 maths enthusiasts got together to write a free (CC-BY) textbook in a weekend. It's the process that's interesting here, not the open textbook angle. Taken from software development notions like hackathons or codesprints, these book sprints are excellent ways of getting books written (Phil Barker (JISC CETS) who shared this info was part of one to write a book on technology for OER).
Is this something that would work for a wider range of disciplines?
Via Phil Barker