If you haven’t read any of Dr. Maffetone’s books on endurance running, you probably should. Here are a few:
The Big Book of Endurance Training and Racing.
The Sub-Two-Hour Marathon Is Within ReachHere’s How It Will Go Down, and What It Can Teach All Runners about Training and Racing
Read his scientific article on heart rate training below or download the PDF here.
Source: The MAF Heart Rate White Paper – Dr. Phil Maffetone