Can't recommend that book enough. Another classic is Tracy Fullerton's Game Design Workshop. I like to read up on cognitive/behavioral psychology for some more outside-the-box perspetive on design; any ole' textbook is a great place to start. Economics is also a good place for some interesting/applicable studies. Good game design is basically psychology applied artfully, so anywhere you can observe humans functioning within a system is a good place to learn.
If you're in the mood for vids (and giggles), check out egoraptor's "Sequelitis" series on youtube; it's in the same vein as Extra Credits, but with some interesting points of its own. Language is definitely NFSW, though, so consider yourself forewarned
Only other advice I could give is to play lots of games critically (not just digital games either!), and make lots of your own games to get in the practice (again, not just digital games).