Your first stop should be the www.postgresql.org website and their user documentation including the pretty comprehensive faq.
There are several mailing lists that may help you.
Bruce Momijan has written a book "PostgreSQL: Introduction and Concepts" which is online here and available for sale in (as they say) all good bookstores. Recommended if you're new to Postgres or SQL.
Other books are in the pipeline, and will no doubt be mentioned on the postgresql website when available.
If you think there is a gap in the Postgres documentation please feel free to volunteer (contact the pgsql-docs mailing list). If you feel something is missing from this document, just let me know what. Likewise, if you would like to translate this document feel free but please check the pgsql-docs mailing list first to avoid duplicating effort.