There are lots of books on MySQL, but few on PostgreSQL. This book is excellent, it covers the SQL language for people who are completely new to databases, and also tells how you can use Pg/SQL, C, C++, PHP and Perl to interact with the database.
It covers installation, creating databases, tables, users, functions, and all other forms of administration, like analysing the tables, optimising queries and backing up and restoring data. I found this a very thorough read, which complements the online PostgreSQL manual very well.
If you have only one PostgreSQL book in your collection, I would strongly advice that this is one of them.