The PostgreSQL docs are a great place to learn about Postgres, and the PostgreSQL Tutorial is the place to start.
General introduction to PostgreSQL for beginners
Covers fundamentals of setup, general SQL, backups, common tools, and Postgres specific tips. Also covers advanced features like HStore, arrays, JSON, and understanding performance.
Tutorials for EnterpriseDB's Postgres Plus Standard Server product apply to PostgreSQL as it is the same server, just with different packaging.
Configuring Apache Authentication with PostgreSQL
- PostgreSQL for beginners
- PostgreSQL Tutorial in detail
- Part 1: Getting Started With PostGIS: An almost Idiot's Guide: Walks you thru how to configure PostGIS, Load data, and do common spatial queries.