This manual set is organized into several parts:
An introduction for new users. Does not cover advanced features.
General information for users, including available commands and data types.
Advanced information for application programmers. Topics include type and function extensibility, library interfaces, and application design issues.
Installation and management information. List of supported machines.
Information for Postgres developers. This is intended for those who are contributing to the Postgres project; application development information should appear in the Programmer's Guide. Currently included in the Programmer's Guide.
Detailed reference information on command syntax. Currently included in the User's Guide.
In addition to this manual set, there are other resources to help you with Postgres installation and use:
The man pages have general information on command syntax.
The Frequently Asked Questions (FAQ) documents address both general issues and some platform-specific issues.
README files are available for some contributed packages.
The Postgres web site might have some information not appearing in the distribution. There is a mhonarc catalog of mailing list traffic which is a rich resource for many topics.
Postgres is an open source product. As such, it depends on the user community for ongoing support. As you begin to use Postgres, 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 it.
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 pgsql-docs (archive) mailing list is the place to get going.