As dumb as this sounds, LiE's advice is pretty much perfect: nail your brotein, and then chow down on EVERYTHING (fats first, then carbz).
Making mass gains effectively is a lot *harder* than some people make out because the sheer quantity of food required for the uninitiated person starting at about 70-80kg is quite surprising. Particularly if you're lifting five times a week and cardio-ing another two...
If you like nuts, then it's quite easy (but expensive)... but milk - by itself - is actually a good source of stuff, too.