The best? The short list:
Frederick Hatfield, Ph.D., "Power"
Dr Philip Maffetone, "Training for Endurance"
Stu Mittleman, "Slow Burn"
Timothy Noakes, "Lore of Running"
John Humphreys, Ron Holman, "Focus on Middle-distance Running"
McArdle and Katch, Williams & Wilkins, "Exercise Physiology"
Peter Janssen, "Lactate Threshold Training"
Pat O'Shea, "Quantum Strength & Fitness"
Michael Yessis, "Secrets of Soviet Sports Fitness & Training"
John Jesse, "Wrestling Physical Conditioning Encyclopedia"
Tudor Bompa, Ph.D., "Theory and Methodology of Training"
James Radcliffe, Robert Farentinos, "Plyometrics"
Jack Wilmore, David Costill, "Physiology of Sport and Exercise"
John Hoberman, "Mortal Engines - The Science of Performance and the Dehumanization of Sport"