Lots of interesting lessons from successfully retired people. Common themes include staying active (volunteering for causes you're passionate about, part-time work that you enjoy), having a strong social network (having good family relations, spending time with friends, continuing to make new (especially younger) friends), life-long learning, and keeping healthy.
The first chapter, "What Will You Do When You Retire?", warns against retiring without having pondered the question. Most people assume they'll enjoy having endless free time to play golf, fish, or whatever else, but quickly end up bored or disappointed. The author suggests it's important to develop hobbies and discover activities you're passionate about in advance.
Chapters are separated by the author's interviews with successful retirees, discussing their day-to-day life and any advice they might have. The stories are varied, yet share some interesting commonalities - most live simply and frugally, de-emphasize financial matters, stress the importance of making younger friends, and many say they feel they barely have time to spend on all their interests (volunteering, working part-time, traveling, hobbies).
There are also many chapters on financial matters, which I quickly skimmed. The information seemed sensible, but I've already read a lot about the financial side of retirement, and found the first half of the book much more interesting.
All in all, it's given me a lot to think about, and gotten me to look forward to retiring.