If you are looking for a book on Postgresql the Douglas book http://www.amazon.com/PostgreSQL-Developers-Library-Korry-Do... is pretty good introduction to most of the topics specific to postgres, it includes a sample implementation of a postgres accessing script in most of the languages commonly used for the task, which can be a bit repetitive (perl, python, php, java, C, same tune different instrument) but that does make it a fairly good reference.

Thought I was add that the PostgreSQL documentation is really, really good for learning. It is a bit dense but overall very well written, includes nontrivial examples, and you can really get a solid dive into a topic reading it.

The current 8.3 documentation is here: http://www.postgresql.org/docs/8.3/static/

I especially enjoy the sections on indexes: http://www.postgresql.org/docs/8.3/static/indexes.html