I just want to second the postgres docs, they're excellent.
They are complete, comprehensive, and well worth your time.
I also like PostgreSQL 9.0 High Performance by Gregory Smith.
Talking about books, I'd say PostgreSQL Server Programming is especially worth to look at, because extensibility is the very thing where PostgreSQL shines.
I had a street fighting knowledge of Postgres. I picked up "Postgres Up and Running" which filled in a lot of holes in my knowledge. It was surprisingly good for an O'Reilly book.