The main documents for each PostgreSQL release are available at http://www.postgresql.org/docs/manuals/.
The most frequently accessed parts of the documents are as follows:
- SQL command reference, as well as client and server tools reference: http://www.postgresql.org/docs/current/interactive/reference.html
- Configuration: http://www.postgresql.org/docs/current/interactive/runtime-config.html
- Functions: http://www.postgresql.org/docs/current/interactive/functions.html
You can also grab yourself a PDF version of the manual, which can allow easier searching in some cases. Don't print it! The documents are more than 2,000 pages of A4-sized sheets.