The PostgreSQL documentation is organized into several books:
An informal introduction for new users.
Documents the SQL query language environment, including data types and functions, as well as user-level performance tuning. Every PostgreSQL user should read this.
Installation and server management information. Everyone who runs a PostgreSQL server, either for personal use or for other users, needs to read this.
Advanced information for application programmers. Topics include type and function extensibility, library interfaces, and application design issues.
Reference pages for SQL command syntax, and client and server programs. This book is auxiliary to the User's, Administrator's, and Programmer's Guides.
Information for PostgreSQL developers. This is intended for those who are contributing to the PostgreSQL project; application development information appears in the Programmer's Guide.
In addition to this manual set, there are other resources to help you with PostgreSQL installation and use:
The Reference Manual's pages in the traditional Unix man format. There is no difference in content.
Frequently Asked Questions (FAQ) lists document both general issues and some platform-specific issues.
README files are available for some contributed packages.
The PostgreSQL web site carries details on the latest release, upcoming features, and other information to make your work or play with PostgreSQL more productive.
The mailing lists are a good place to have your questions answered, to share experiences with other users, and to contact the developers. Consult the User's Lounge section of the PostgreSQL web site for details.
PostgreSQL is an open-source effort. As such, it depends on the user community for ongoing support. As you begin to use PostgreSQL, you will rely on others for help, either through the documentation or through the mailing lists. Consider contributing your knowledge back. If you learn something which is not in the documentation, write it up and contribute it. If you add features to the code, contribute them.
Even those without a lot of experience can provide corrections and minor changes in the documentation, and that is a good way to start. The <email@example.com> mailing list is the place to get going.