Both the Green and Blue book are good. Gordon writes well, so he gets his points across nicely. Green book is pretty comprehensive, but consequently on the more general side. That being said, it was the first and only book I had my wife read on NL. Blue book is play by play of hands.
Both are recommended.
If you don't want to go that route, I recommend the "Killer Poker" series (all of them) by Vorhaus.