"I KNOW HOW SHE DOES IT: How Successful Women Make the Most of Their Time" by Laura Vanderkam (Portfolio/Penguin) $27.95, 304 pp

Time passes, writes Laura Vanderkam, whether we choose what to do with it or not.

Vanderkam is a journalist and time-use expert whose new book is a refreshingly optimistic take on a topic rarely approached with a sunny outlook: having it all.

Vanderkam recruited 133 women who earn more than $100,000 per year and have at least one child under 18 living at home and asked them to track how they spent every hour for a week.

"Everyone has opinions on having it all," she writes. "I want to show, moment by moment, how it's really done."

I love the book for its optimism but also for its powerful, empowering message, woven throughout each chapter, that we are the directors of our own lives.

"Many of us have space to lean deeper into our careers if we wish, but stories and assumptions -- that leaning in will require harsh trade-offs -- have great power over our lives," Vanderkam writes. "When it comes to work, especially for people who are trying to build families, these stories can limit our lives and our careers in profound ways. But we can change our stories if we want." The book is especially instructive for anyone at a career crossroads, deciding what the next professional chapter should look like.

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"What if you start with the assumption that there is enough space to do all of the things that are important to you?" Vanderkam said. "What if you told yourself it is all possible?" Vanderkam, whose four kids range in age from 4 months to 8 years, championed the use of time logs in her 2011 book, "168 Hours: You Have More Time Than You Think" (Portfolio).

"Years ago, when I filled out my first 168-hour (one week) time log, I thought that it seemed strange to view life as cells on a spreadsheet," she writes. "But over time I came to see that I could view myself as the artist deciding on those cells. I became a mosaic maker, carefully placing tiles."

I know, I know. Women making more than $100,000 a year have a lot more freedom over their tiles than the rest of us. And, as Vanderkam points out, less than 4 percent of women employed in the U.S. earn six figures.

So why focus on them? Partly, Vanderkam contends, to show how women with demanding, high-powered jobs find time for kids and partners and hobbies. And partly, she told me, to inspire others.

"I hope young women don't fear the big job," she said. "You want the big job. The big job is going to make life more doable. And big jobs are often quite fascinating -- you have autonomy, you make interesting decisions, you use your creative capabilities."