I'm a huge fan of Positive Psychology and have read dozens of books (by Ph.D researchers of the subject).
My favorite academic summary is The Good Life: Unifying the Philosophy and Psychology of Well-Being by Michael A. Bishop. As a hyper-short summary: psychologists studying well being have used different definitions and approaches, yet what unifies them is positive feedback loops - behaviors and activities that beget more positive affect in the future. Think spending time with good friends, etc.
https://www.goodreads.com/book/show/22897962-the-good-life
A classic read is Stumbling on Happiness by Daniel Gilbert. It shows how humans are systematically bad at predicting to what extent future events will make them happier or sad. His TED talk is a nice 20 minute summary of the book.
https://www.goodreads.com/book/show/56627.Stumbling_on_Happi...