The introduction tutorial is concise but more than sufficient.
The reference guide is well written, laid-out and covers everything you need:
the database server, its configuration and tools, the SQL language, its operators, data types and grammar, the built-in procedural languages, the low-level client library, and all the database internals (system tables, data file formats, network protocol, query optimiser, indexes, etc).
I'd second this. I've pretty much fallen in love with the official docs over the last few months. They remind me of perldoc in terms of clarity and general quality.
The mailing lists also seem to carry a lot of good material.
The introduction tutorial is concise but more than sufficient.
The reference guide is well written, laid-out and covers everything you need: the database server, its configuration and tools, the SQL language, its operators, data types and grammar, the built-in procedural languages, the low-level client library, and all the database internals (system tables, data file formats, network protocol, query optimiser, indexes, etc).