Postgresql documentation is terrific, really [1]. Then you can also use pgsql wiki [2]. Other useful resources are the searchable mailing lists archive [3]. Have a look also to pgcon.org web site, it's fully packed of presentation material (mainly pdf slides) gathered since 2007, just to give you an example look at Tom Lane's "http://www.pgcon.org/2011/schedule/events/350.en.html [4]

Speaking of books a few of the best I'm aware of are:

- PostgreSQL 9 Admin Cookbook (Simon Riggs, Hannu Krosing)

- PostgreSQL 9.0 High Performance (Gregory Smith)

- Instant PostgreSQL Backup and Restore How-to (Shaun M. Thomas)

all them from Packt Publishing

[1] http://www.postgresql.org/docs/devel/static/

[2] http://wiki.postgresql.org/wiki/Main_Page

[3] http://www.postgresql.org/list/

[4] http://www.pgcon.org/2011/schedule/events/350.en.html