One of the best resources for PostgreSQL is the online documentation - I've hardly seen a F/OSS project with a better written documentation. The only drawback is that it is quite verbose and it's easy to get lost. You might want to start with the official tutorial [1] and might also check the dated "MS-SQL to PostgreSQL Migration" (from 2001) [2] which still provides some nice transition tips.

Make sure to check the "Postgres Guide" [3] which is a recent addition and is excellent to get you started quickly.

The quick intro by Packt Publishing [4] looks useful and concise as well, but is not as good as the Postgres Guide.

Since you're familiar with the RDBMS concepts I would suggest you take a look at the nice "PostgreSQL 9 Admin Cookbook"[5] which has a solid collection of recipes that you can use to learn the specifics of the DB and a bunch of nice features.

[1] http://www.postgresql.org/docs/9.2/static/tutorial.html

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

[3] http://www.postgresguide.com/

[4] http://www.packtpub.com/article/introduction-to-postgresql-9

[5] http://www.amazon.com/dp/1849510288/

edit: fixed the link to postgres guide.