If that pink O'Reilly book "Practical PostgreSQL" is the most recent PostgreSQL book on your shelf, it's time for an upgrade to Neil Matthew and Richard Stones' second edition of Beginning Databases with PostgreSQL. I kind of hesitated to pick this one up since it sounded like it might be too much of a beginner's book, but I've been very pleased with it. After a couple of introductory chapters it gets right down to business with a good discussion of psql, advanced querying (correlated subqueries and outer joins and whatnot), and fine sections on triggers and stored procedures.

Most of the PostgreSQL work I do (on indi) is using Ruby and the ActiveRecord object/relational mapping library, so I don't do much actual SQL or querying PostgreSQL from C. But it's nice to know that the book includes a chapter on libpq in case I ever need to hack around the Ruby PostgreSQL extension.

Anyhow, I highly recommend this book; it's been very useful for me. Now I just keep it on my desk instead of always keeping a browser window open to the online PostgreSQL docs :-)