There is also PostgreSQL: Introduction and Concepts" by Bruce Momjian ISBN 0-201-70331-9.

The Postgres docs are good. Recommended.

For those who like hard copy get a mid-range laser printer (us$500) which prints at a cost around 2 cents per page. It will pay for itself in about the first 25 technical books that you don't buy. Perhaps sooner if it leads you to read more on online also saving the cost of printing.