We use both Oracle and PostgreSQL at work; there isn't a massive choice of books on PostgreSQL, but I felt it was worth looking for something to bring me up to speed on its quirks and on the administrative side of things in particular.
This is a big book, and it covers a lot of material well - I've learnt quite a few things as I go that answer some of my questions and give me pointers on others. The first section deals with some standard material such as SQL syntax, but also covers PostgreSQL-specific data types and extensions, and the query optimizer. The second section covers client access, with the same basic tasks accomplished in many languages. The third section covers miscellaneous tasks such as system administration, backup, security, i18n, replication and additional contributed modules.
Unfortunately, the book suffers from many typos and formatting problems, and in the client access section, the chapter on Java, the language with which I am most familiar, the code examples forget to close() any database objects once they're done, which is poor programming practice. It leaves me with the overall feeling that this is a useful book, but it's difficult to regard it as especially authoritative.