The fundamental premise of this book is your basic: "you can't choose what happens, you can only choose how you react." Happiness is a choice. A simple but very profound principle and one that is much easier to "believe" than it is to "employ".
Rubin lists a series of philosophers and other thinkers and researchers who have studied "happiness" from a variety of perspectives and historical periods, and she references some of their discoveries/conclusions, but her researches are background, not foreground. Instead of attempting some kind of summary or new perspective on existing "knowledge" she undertakes something equally if not more interesting, which is to systematically apply what she has learned and evaluate results in real life, her life.