Among the more powerful changes in planning for retirement is the embrace of encore careers, mostly part time but sometimes full time.

In the vanguard of the movement of working during the traditional retirement years (really, the unretirement years) are people who would like to continue using their accumulated knowledge and skills. They'd often want to shift into a job that offers them greater emotional and psychological rewards, a job that offers meaning and purpose. Earning an income also boosts household economic security.

For anyone thinking about what comes next, I highly recommend reading Lucy Kellaway's "Re-educated: How I changed my job, my home, my husband & my hair." As you can tell from the title, Kellaway made more than the usual number of shifts in her encore.

She taps into each transition to smartly illustrate the challenges, the opportunities and the learning that come from trying a new career later in life. The book is a delightful read. Her insights are far more useful for those launching their late-life transition than most how-to books.

Kellaway had a career she loved writing a column for the Financial Times. She found herself restless following the death of her mother and later the passing of her father. She realized it was time to make a change.

"I don't think I was especially depressed and there was no fizzing in my head; I was just unhappy and monumentally stuck," she writes. "I had been doing the same thing for too long and was worn out mentally, physically and emotionally."

She quit her job at age 57. She amicably divorces her husband, buys a dilapidated home she fell for, co-founds a nonprofit, and lands a job teaching economics to teenagers in one of the poorer parts of London. She stops dying her hair.

Kellaway is now working harder than she did at the Financial Times. She is constantly trying to be a more effective teacher. She also came to realize that she hadn't reinvented herself despite all the transitions.

"I now see what should have been obvious all along—what changed is not my character but my experiences. I am immersed in a new world that feels a long way from my old one," she writes. "Though I have not been reinvented, what has happened is just as radical and a lot more interesting. I am being re-educated."

She found both purpose and a paycheck in her unretirement.