The myth of Sisyphus pops up in Tracy Kidder's "Rough Sleepers," about a doctor who works with the unhoused population of Boston, and it's an apt one. The book's subject notes that Sisyphus, always striving toward a goal he'll never reach, is usually thought of as eternally frustrated. But what if Sisyphus is energized by the challenge? What if the striving is what he seeks?

That may be a rationalization for Dr. Jim O'Connell, who has spent decades finding medical care and housing for people, knowing that many will backslide and dozens more folks in need will turn up the next day. But it seems to work for him.

Frequently referred to as "Saint Jim" — although never by himself — O'Connell left Harvard Medical School and detoured into what he thought would be a year of caring for the unhoused, only to discover it was his life's work. Now in his 70s, he's still "making housecalls to the homeless," as CBS News put it in a profile.

O'Connell is a fascinating protagonist. Kidder spent years shadowing the indefatigable doctor on the job, examining patients under bridges, handing out $20 bills in public parks and begging the sick to go to emergency rooms (as well as the Legislature for funds).

There's evidence that Kidder dug for behavior to contradict the "Saint Jim" moniker but the best he comes up with is hearing O'Connell swear once (a marriage ends somewhere along the way, but we've never met her and don't know what happened). The picture that emerges over the course of the absorbing, inspiring "Rough Sleepers" is that O'Connell is not only one of the good guys but a good guy who is vigorous, self-critical and even funny.

About that mystery marriage, though. Kidder's best books, including "House," "Among Schoolchildren" and "The Soul of a New Machine," have built-in timelines: creating a home from design to move-in, a year in the life of a classroom or a deadline for designing a computer. Absent them, Kidder is not great at structure.

It's difficult to gauge the chronology of "Rough Sleepers," which spans several decades. Longtime O'Connell colleagues pop up two-thirds of the way through the book and it seems like we're supposed to know who they are but we don't. Others vanish with no explanation, with O'Connell's second (I think?) wife an especially confusing cipher.

It may be that Kidder's deal with O'Connell was not to intrude on his private life, which is probably for the best. Actually, it's hard to imagine O'Connell having any time for a private life, given all of the 18-hour days that Kidder describes with modesty and compassion in "Rough Sleepers."

Chris Hewitt is a Star Tribune features writer and critic.

Rough Sleepers

By: Tracy Kidder.

Publisher: Random House, 320 pages, $30.