Necessary Endings: The Employees, Businesses, and Relationships That All of Us Have to Give Up in Order to Move Forward by Henry Cloud
This book is based on the premise that endings are needed in order to move on in work and life in general. The book goes through reconsidering how we think about endings, how to decide whether something can be saved and is worth saving, and how to end well.
Practical and easy to read, but a lot more geared toward the workplace although there is some talk about personal relationships.
Helpful, especially if you're someone who avoids endings or thinks endings are negative.