I've started with the book "PostgreSQL Up & Running". It's rather lacking in detail and has a lot of typos, but I'm nevertheless enjoying it. It's written in a simple language and it provides a good landscape overview, which is exactly what you need in the beginning. Get the big picture first and then you can look for the necessary details as you go.

http://shop.oreilly.com/product/0636920032144.do

The recently released 'PostgreSQL: Up and Running' may be right up your alley, at only 168 pages:

http://shop.oreilly.com/product/0636920025061.do

Our site is dedicated to that: http://www.postgresonline.com/

Also check out our new book which came out this month. http://shop.oreilly.com/product/0636920025061.do

Its primary target audience are people coming from other relational databases.