Ive have both super system I and II and to be honest I think there crap. Actually I think thats harsh but there just not nearly as good as other books. Doyle's no limit parts is all about high stakes NL. He continously refers to pots being in the $10000 range and him having an endless bankroll.
If you play high stakes and are willing to gamble alot then it might be good for you other than that id recommend sklanskys books. Much more down to earth and realistic as well and shows in depth stuff that exact pot odds and reasons behind certain plays (its a tough read but i think its worth it).
Just to give you the jist of doyle brunsons super system NL section. Dont be aggresive be insane aggressive so aggressive that youll be willing to go into a pot with the worst of it (notic i said aggressive three times). Thats basically it just bet and raise as much as possible.
Thats just my opinion though i seriously think super system is over rated to the average player.
Also i was just reading an article on this site by howard lederer saying that playing limit holdem will benefit you in NL as well as it will force you learn odds and post flop play. Ive read tons of limit books but mostly play NL (not online) and i can tell you its helped my game alot, just remeber theres a difference.

And one more thing Id always like to see AA over AK in NL anytime i dont care what doyle says.