To answer that question, longtime Morningstar columnist and podcaster Christine Benz asked 20 retirement thought leaders to go deep on a single lesson that they believe contributes to success in retirement.

These lessons range from nitty gritty financial matters to quality-of-life considerations that help pre-retirees and retirees maximize their "time on earth" allocations.

On the financial side of the ledger, the book delves into knowing your retirement income style (Wade Pfau), understanding how your spending might change in retirement (David Blanchett), and organizing your portfolio to support in-retirement cash flows (William Bernstein).

Other interviews probe softer but no less important considerations, such as how to spend in order to optimize happiness (Ramit Sethi), the value of burnishing relationships later in life (Laura Carstensen), and living life so that you have no regrets in the end (Jordan Grumet). These lessons help soon-to-retire and already-retired individuals and their advisors tackle retirement with confidence, wisdom, and a specific plan for maximizing their financial and human capital.