PostgreSQL includes extensive documentation, including a large
manual, manual pages, and some test examples. See the /doc
directory. You can also browse the manuals online at http://www.PostgreSQL.org/docs.

There are two PostgreSQL books available online at http://www.postgresql.org/docs/books/awbook.html
and http://www.commandprompt.com/ppbook/.
There are a number of PostgreSQL books available for purchase. One
of the most popular ones is by Korry Douglas. A list of book reviews
can be found at http://techdocs.PostgreSQL.org/techdocs/bookreviews.php.
There is also a collection of PostgreSQL technical articles at http://techdocs.PostgreSQL.org/.

The command line client program psql has some \d commands to show
information about types, operators, functions, aggregates, etc. - use \? to
display the available commands.

The PostgreSQL web site contains even more documentation.