When Michael Clark shipped out to the Vietnam War with the Navy, his superiors told him he could bring 120 pounds of personal possessions.

Roughly two-thirds of Clark's allotment -- about 85 pounds' worth -- was books.

"I spent four years in the Navy reading all the books I was supposed to read in college," Clark said.

Despite his love for classic tomes, Clark bypassed a career working with books and became a newspaper journalist. In his 25 years in the business, he wrote for newspapers in California and Nevada, and even owned Nevada Senior World in the 1980s and '90s.

But Clark began a new chapter in his career in 1995. At 48 -- when many people are beginning to weigh whether they'll have the savings to retire in a decade or so -- Clark sold Nevada Senior World and put himself out of a day job. It was the perfect opportunity to transform his favorite hobby into a way of making a living.

"I told my wife, 'I have a chunk of money right now and I don't know what to do with it,'" Clark recalled. "She said, 'You already have about 3,000 books. Why don't you go ahead and be a book dealer? You always wanted to do that.'"

And so Clark opened Michael's Books and Movies on East Tropicana Avenue near Pecos Road. Now, at 59, he's part of a generation of older workers exchanging a quiet, idle retirement for meaningful positions in part-time work, consulting and even first-time business ownership.

"We're going to see a different type of retirement than we've seen in the past," said Deborah Moore, director of communications for the Nevada chapter of AARP. "We're already seeing an evolution in retirement as baby boomers once again redefine a life stage."

Nevada's AARP chapter surveyed 2,000 of its members in late 2006 and found that 32 percent of them said it's highly likely they'll continue to work during the years traditionally reserved for retirement.

Among those who will probably work, 61 percent cited needing or wanting the extra income as the top reason they'll continue to punch the clock. Maintaining health insurance was motive No. 2 for holding down a job in the retirement years, with 49 percent of respondents ranking coverage as an important factor in working. Protecting savings came in at No. 3, with 47 percent of respondents calling it a priority. And 42 percent said they would work into their dotage simply for the enjoyment of the job.

It's a demographic wave experts say employers should consider riding, given worsening labor shortages in sectors such as health care, education and the sciences.

Moore said the share of people 50 and older in America's labor pool will reach 20 percent in 2012, up from 12 percent in 1992.

"We need to figure out ways to keep as many people as possible in the work force, and make sure they're engaged, happy, active and healthy," Moore said. "It makes good sense for businesses, it makes good sense for government and it makes good sense for individuals."

A love for work lured Las Vegans Richard and Jo-Ann James back into the employment fold after retirement.

The couple in 2003 left lifelong careers in upstate New York, where he was a food inspector and she was a secretary to a school-district superintendent, and moved to Las Vegas. Jo-Ann James took to reading "help wanted" ads in the newspaper to get a feel for the local economy. In 2004, she spotted a notice seeking part-time convention workers; intrigued, she and her husband decided to give trade-show service a try.

"It sounded so interesting," she said. "A few days a month (working part time) would be perfect, so we could get to know the community and do something a little bit different."

Nearly three years later, the couple is still at it, putting in anywhere from three days to 10 days a month registering conventioneers, monitoring check-in lines, distributing badges and serving inside information booths, among other tasks.

"It's just nice to get out of the house and meet people," Richard James said. "You feel like you're doing something for yourself, rather than just sitting home and doing nothing."

Working the convention beat is also considerably less stressful than their former jobs, Jo-Ann James said. Trade shows have definitive ending dates, making it tough to take on the added responsibilities that can turn a part-time lark into a full-time grind. The Jameses use their earnings to subsidize travel or buy their family tickets to shows on the Strip. They occupy their down time with philanthropic work, stuffing envelopes that the Department of Homeland Security will send to new citizens, reading poems at citizenship ceremonies, helping people over 50 with their taxes through AARP and volunteering at McMillan Elementary School, where their daughter is a librarian. Working part time suits their lifestyle.

"If we want to go away on vacation, we just refuse the job," Jo-Ann James said. "If we'd rather volunteer (than work), we just say no to a job."

Lassoing more retirees into office duty requires attention to the demographic group's priorities.

AARP's Nevada survey found that a flexible work schedule is the most important characteristic to post-50 workers, with 62 percent of respondents saying that the ability to set their own hours would be highly likely to lure them back to work. Following adjustable hours were incentive pay (40 percent), training opportunities (28 percent), phased retirement positions with successively fewer hours (27 percent), job sharing (25 percent) and advancement opportunities (23 percent).

Companies in the market for seniors should begin their hunt within their own staffs, Moore advised. It can cost 50 percent or more of annual salary to replace a worker, so rather than releasing a senior to retirement, consider convincing him to stay on part time.

"Everybody's path is different, so people want flexibility," Moore said. "And we need to dispel the myth that (older workers) are inflexible about learning. They're looking to be retrained and they're looking for new opportunities."

For Rose Johnson, the ability to set her own schedule was enough to encourage a return to work.

Johnson was a nurse for 35 years, retiring in 2000 from Sierra Health Services at 62. Two years later, Sierra Health called Johnson with a fresh offer: Come back on a part-time basis with four other nurses to perform quality-assurance reviews, and work as often as she wanted for up to 1,000 hours a year. Johnson now puts in four or five hours a day, three to five days a week, reviewing patients' charts to see if they're being followed properly according to specific health or treatment measures.

"I can't say I went back to work because I was getting bored," said Johnson, who's now 69. "I was doing lots of volunteer work. But I love Sierra Health. It was a good company to work for, and they asked me to come back. The fact that they gave me a choice of how often I could work was important to me. That flexibility of doing something you already know, of doing it when you want to do it, of calling the shots and making money for it -- it's great."

Johnson said employers would be wise to consider hiring more seniors and retirees.

"If seniors are going to be given a second chance to do something they already know, they're going to do it well," she said. "They want to be good at what they do, and they already know the job. They're dependable."

Moore agreed.

AARP research has found that older workers are more motivated than their younger counterparts to exceed expectations on the job, Moore said. After all, seniors are investing their discretionary time into work, which can mean a higher level of engagement on the job.

"And businesses with more committed employees often have better bottom lines," she said.

Ideally, Moore said, postretirement careers will offer a mix of work and fun.

It's a formula Clark, the bookseller, appears to have mastered.

"Every day in the bookstore is like Christmas," Clark said. "Every day, you see something you've never seen before, no matter how many years you've been doing it and how many thousands of books you've gone through."