Ask HN: good book or resources to get my SQL skills to the next level

I'm doing web development for about 3 years now (mainly ruby stuff), used SQL all along and before (I was doing C/C++ and sqlite dev). I know basic SQL, joins and all basic SQL querying. But I'm pretty sure there's tons of post processing code I could write directly in SQL. I mean, I know I can so ... do you have a good book to recommend to get my SQL skills to the next level ?