I do not know whether it is the best text on the subject but it is a text that made a powerful impression on me: “The Power of Now” by Eckhart Tolle.
It is my assumption that “well-being” is a state of mind much more than a physical state.
I often observe that people either re-live events of the past (often the unhappy ones) or imagine and worry about events that are about to happen. I found for myself that it is very unhelpful to worry about things that have not yet happened. Even on a physiological level, that is harmful because our body responds to imagined stress as much as to real one. That is why I believe that I can enhance my sense of wellbeing by the thoughts I choose to think. Whether this is a strategy for somebody else …… I like to think so.
I recently read a good line about such thinking: “Imagine there are 1000 tigers chasing you and are about to catch you. What would you do?” The answer: “Stop imagining 1000 tigers.”