Jon Kabat-Zinn: Full Catastrophe Living: Using the Wisdom of Your Body and Mind to Face Stress, Pain, and Illness
While not geared specifically toward training, it discusses meditation techniques to help you focus and deal with the discomfort of racing.
Joe Friel: The Triathlete's Training Bible
A great tool for the self-coached triathlete. Covers training plans, weekly schedules, and much more.
Ph.D. Jim Taylor: The Triathlete's Guide to Mental Training (Ultrafit Multisport Training Series)
Pushing your limits is all in your head. Train your confidence, focus and intensity.
Terry Laughlin: Total Immersion: The Revolutionary Way To Swim Better, Faster, and Easier
A new way to think about swimming--swim easier and go faster.
Hunter Allen: Training and Racing with a Power Meter
Pretty much the authoritative source on the topic.