From Social Patterns
Where traditional books are released in finished editions, each building on its predecessor in small, incremental ways.
An Unbook in contrast is more like software - a continuous work-in-progress where authors (more like collaborators) come together to continue to build this resource on an ongoing basis. An Unbook is released in versions, just like software. Version n.0 is a significant imporvement, with enough improvements to be released as a new version. Borrowing a leaf from standard configuration management practices, the gap between two successive releases of the Unbook is an indicator of the significance of a new release. For example, the difference between 1.1 and 1.1.3 is minor, while the difference between 1.1 and 2.0 is major.
An Unbook exists because of the support by a community of users who share their experiences and best practices with each other, and help each other troubleshoot problems encountered in their practice areas.