Found in 3 comments on Hacker News
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.

mattyb · 2012-10-02 · Original thread
The recently released 'PostgreSQL: Up and Running' may be right up your alley, at only 168 pages:

robe2 · 2012-07-22 · Original thread
Our site is dedicated to that:

Also check out our new book which came out this month.

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

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