There are several books out there on PostgreSQL. Here are a few:

PostgresSQL Essential Reference, by Barry Stinson, New Riders Publishing, ISBN ISBN 0735711216

PostgresSQL, by Susan Douglas & Korry Douglas, New Riders Publishing, ISBN ISBN 0735712573

Practical PostgresSQL, by John C. Worsley & Joshua D. Drake, O'Reilly, ISBN ISBN 1565928466

The first book in this list I have used extensively and have found to be quite useful. You should be able to order all of these at any major book store.