The Big Book of PostgreSQL by Thom Brown.
From the post:
Documentation is crucial to the success of any software program, particularly open source software (OSS), where new features and functionality are added by many contributors. Like any OSS, PostgreSQL needs to produce accurate, consistent and reliable documentation to guide contributors’ work and reflect the functionality of every new contribution. Documentation also an important source of information for developers, administrators and other end users as they will take actions or base their work on the functionality described in the documentation. Typically, the author of a new feature provides the relevant documentation changes to the project, and that person can be anyone in any role in IT. So it can really come from anywhere.
Postgres documentation is extensive (you can check out the latest 9.4 documentation here). In fact, the U.S. community PDF document is 2,700 pages long. It would be a mighty volume and pretty unwieldy if published as a physical book. The Postgres community is keenly aware that the quality of documentation can make or break an open source project, and thus regularly updates and improves our documentation, a process I’ve appreciated being able to take part in.
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A recent podcast, Solr Usability with Steve Rowe & Tim Potter goes to some lengths to describe efforts to improve Solr documentation.
If you know anyone in the Solr community, consider this a shout out that PostgreSQL documentation isn’t a bad example to follow.