Going Long I haven't actually got but The Triathlete's Training Bible by Joe Friel is a must-read, even if you don't stick with everything he says.
Be Iron Fit (Fink) is good.
If you want to train with power, have a look at the Coggan/Allen book, Training and Racing with a Power Meter (and actually there are a few good articles on cycle-oriented training on Flamme Rouge that are inspired by this).
For diet, either (or both of) The Complete Guide to Sports Nutrition by Anita Bean & Racing Weight by Matt Fitzgerald.
Lore of Running is good despite not covering tri, particularly if you're interested in the physiology of endurance activity.
For swimming, a lot of triathletes use Total Immersion by Laughlan, though personally I've never got on with it. The book is improved a lot by the video.
To get your head in the right place, there's The Winning Mind by Seb Coe. I haven't actually read this but I'm informed of it's brilliance by a guy who actually wins stuff!
Finally for inspiration, get your head into Holgs' book Can't Swim, Can't Ride, Can't Run. Chrissie's autobiography A Life Without Limits and I Came To Win by Macca. A great piece of Iron distance history is covered in depth in Iron War by Matt Fitzgerald.
That should keep you going.