The PostgreSQL documentation. http://www.postgresql.org/docs/9.3/static/index.html

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).