In 2003, I read Po Bronson's book, What Should I Do With My Life? as I was contemplating the second act of my career. The book profiles 50 individuals as they search for their true calling in life.
It was very interesting to me to learn the challenges of others as they made a journey similar to mine. Those stories were very useful to me as I made the work transition to non-profit industry in 2004.
If you're thinking about your second act and are unsure what to do next, I'd encourage you read this book as your first step. Understanding how others have crossed the same bridge is eye-opening.