(self plug) for learning Postgres-flavoured SQL, take a look at https://pgexercises.com.

Books-wise, Learning SQL is a great start. http://shop.oreilly.com/product/9780596007270.do

The postgres docs themselves are also pretty fantastic.

Depends on what you're into, but from the database angle:

Learning SQL is a really excellent intro, and probably covers the majority of what your average developer is going to ever need to know about SQL ( http://shop.oreilly.com/product/9780596520847.do )

SQL and Relational Theory gives you more background on relational theory - basically gives a more academic background for existing practitioners of SQL. Date is pretty dry author with a few hangups, but it's largely good solid stuff ( http://shop.oreilly.com/product/0636920022879.do )

Oracle Essentials is a good solid introduction to Oracle, and gives a useful bit of background on how database management systems work ( http://shop.oreilly.com/product/9780596514549.do )

For other subjects:

Code Complete needs little introduction ( http://shop.oreilly.com/product/9780735619678.do )

I've only just started reading Version Control with Git, but it seems pretty good so far, and is well-reviewed ( http://shop.oreilly.com/product/9780596520137.do )