Super System's value is in explaining several different variations of poker... as in all of them. It's only a starting point because every variant has evolved, even past the information updated for SSII. But as a starting point, you won't find a better book for getting into mixed games.
If you're trying to focus solely on NLHE, you could easily skip Doyle's books and instead start with Gordon's Little Green and Blue books (haven't read Gold yet) and then move to one of Harrington's series depending on whether you're more of a cash game or tournament player. Daniel Negreanu's Power Holdem has a good mix of the two, and I would recommend it even if you're not a small ball player. It will help you defend against opponents using it against you.
I really don't know what you'd go to after that as far as advanced cash game play, but you should check out Jonathan Little's Secrets of Tournament Poker Vol I and II released in the last two years for how to best beat today's poker tournaments. Whether you play live or online, his advice and teaching style can help your tournament game, and it's especially good for making a transition from online to live.