I've borrowed a friend's copy when I needed to do git stuff (we use Mercurial at Blekko) but figured it was time to get my own copy.
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 )
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