I just published a new book on Amazon Kindle, Efficient Heart Rate Training, a short book on why, how and for what benefits you should measure your heart rate while training for a run, both with a watch or an iPhone.
Of course, given that I am also the developer of 321Run, I am more prone to advise you to use an iPhone app.
Here is the description
Did you know how much you can benefit from a heart rate measure to better train for running? Even if you are just beginning?
Heart rate training is the best way to know your effort, measure it, know your body and improve your health and well being. Wether you are looking for weight loss or to improve your fitness, it will help you understand where you are, where you need to go, and how you can improve.
This ebook goes into the scientific knowledge that supports heart rate training and explains in common language these facts. It helps you test yourself and define your goals in simple, understandable words. It provides you with the essential knowledge to be able to start planning a regular fitness exercise, improve immediately your current training, and feel better with a few exercises.
The iBooks version is still in review by Apple but should be published in a few days.