A decent book, Practical PostgreSQL by John Worsley(O'Reilly) is available on-line. Although it was written for version 7.1x, many of the points made should remain valid with later editions of PostgreSQL.

PostgreSQL, Second Edition, by Douglas & Douglas. (Sams) is a pretty useful addition to your library. It is broken into three sections, aimed at the three classes of users: The first section deals with using PostgreSQL, and includes Information on PostgreSQL's SQL Syntax and use, working with data, and performance. The second section is aimed at developers, and is primarily concerned with programming with postgreSQL, contains information on programming with PostgreSQLs existing languages, as well as extending PostgreSQL. Finally, the third section is aimed at PostgreSQL administration. Here is Chapter 3: PostgreSQL SQL Syntax and Use, in its entirety.

SQL in a Nutshell, Second Edition, by Kline, Kline and Hunt (O'Reilly) is a comparitive SQL reference. It compares the various dialects of SQL, command-by-command for ANSI SQL, SQL Server, DB2, Oracle, PostgreSQL, and MySQL. There's also a separate appendix dealing with the differences between SQL Server and Sybase Adaptave Server (12.5) Note that this book makes no pretense at explaining the various SQL commands. It assumes that you know at least one of the SQL dialects, and need know the differences between the dialect that you know, and a different implementation.