Quick summary: PostgreSQL Up and Running, by Regina Obe and Len Hsu, tells you everything you need to know if you want to start playing with the database engine that all the cool kids are using. It assumes that you already know something about SQL, but as the title promises, it walks you through how to get a PostgreSQL server installed, set it up, get a login role configured, and start playing. After reading it, you'll know about the cool features that PostgreSQL brings to the party (such as table inheritance, custom types, and extensions), the basics of running a postgres instance, some of the key configuration parameters, areas (like permissions) that have unavoidable complexity, and where to go to learn more.
The PostgreSQL 9.1 reference manual, available for free from [...], is extensive, highly detailed, and large - it's nearly 2700 pages. This can be quite intimidating for a new user, as it offers only a 5-page "getting started" section that doesn't provide much handholding if things go wrong.
"PostgreSQL Up and Running" fills that gap. It's more than a tutorial, but isn't weighted down with the esoterica in the reference manual. The authors have been active in the PostgreSQL community for some time, and have also written a book, PostGIS in Action, on the PostGIS spatial database environment built on top of PostgreSQL. They've got the experience needed to explain the postgres environment with a focus on the useful rather than on the comprehensive.
The book has 10 chapters, in two broad groups. The first group covers basics, database admin, the psql command-line tool, and the pgAdmin GUI. These chapters cover downloading a postgres distribution, getting it installed and running, the basics of configuring and administering it (including extensions, backup and restore), and a couple different ways of talking to it once it's up. This group of chapters offers many sample queries and command invocations, heavily annotated to explain what each part of the query or invocation is doing.
The second group covers what you need to know to write programs against the database engine - data types; table, constraints, and indices; appropriate SQL style for PostgreSQL; writing custom functions; performance tuning; and replication and external data. You may not care about all of those topics, but it's useful to know what your options are. In particular, Postgres offers a variety of data types that may be new, such as arrays and rows; some with more options than you might expect, such as date time; and some where the intuition about tradeoffs may be different than you're used to, such as varchar vs text. The book covers thee common use cases and offers some guidance about common areas where you might get tripped up.
The authors clearly like using PostgreSQL; there are a number of sections in the book that start with a sentence like "PostgreSQL support for is the best of any database we've come across." While this is hard to deny (I don't know what databases they've come across), it's a little distracting. Nevertheless, they usually follow this sort of comment with a clear explanation of how that feature works.
I got my start with PostgreSQL using the combination of the PostgreSQL reference manual and the authors' PostGIS book. I wish that PostgreSQL: Up and Running" had been available then; it would have saved me some time and confusion. If you're curious about postgresql, this book will give you a guided tour by the experts. Recommended, and I'll be buying a couple copies to give to new people on my development team.
Full disclosure: O'Reilly offered me a free copy of the ebook in exchange for a review. Turns out, I'd have bought it anyway.