After retiring at 51, my uncle, Dr. Rob Pascale, wrote The Retirement Maze. It came as a result of how unhappy he was. Here this guy lived on a professional golf course, his kids were doing well, and he had lots of friends, but was miserable.
He finally asked himself why am I so unhappy? and used his money and skills as a research psychologist to self-fund what became the most comprehensive study of its kind.
He's 67 now and doing much better.
The case studies are pretty interesting. One is a police chief who was forced into retirement due to politics. He said "I didn't realize how much time I was spending with my dog until I did a Google street view of my house, and there we were sitting on the porch." That guy got his act together and started solving cold cases until he past away a couple years later. I imagine he brought some real peace to quite a few families.