In his new memoir, “The Line Becomes a River,” Francisco Cantú introduces an uncommon leading man: the U.S. Border Patrol agent with a heart of gold.

That agent would be the author himself, who joined the federal agency after studying the border in college and finding he yearned for an unvarnished look, “not sitting at a computer, not staring at papers.” But Cantú is soon beset by troubling dreams, misgivings about some patrol tactics and empathy for border crossers he sends back south. In an often raw and timely confessional, the former Fulbright fellow and Pushcart Prize winner paints a striking picture of the unsparing borderlands, even as he often finds coarse beauty in the desert terrain where he and his colleagues plied their trade.

Early in the book, Cantú relays a conversation in which his mother, the daughter of a Mexican immigrant, questions his plan to join the Border Patrol, or la migra. She warns him about “stepping into a system, an institution with little regard for people.” He counters that the people he encounters will find in him an officer who speaks their language and has traveled in the places from which they hail — “a small comfort.”

And they do. Cantú listens to the stories of crossers before he processes them for deportation. Once, he gives one his shirt; he washes the blistered feet of another. For some who have wandered the desert for days without water, the agents are not just the guys who arrest and send them back — they are life savers. Cantú and his colleagues try their best to disrupt the business of violent, ruthless men. His cohort, he notes, is nearly half Latino: military veterans, business owners, family men.

But he increasingly struggles with some of the agency’s practices, from slashing water bottles when they come across migrant stashes in the desert to letting the men ferrying seized drug shipments get away to spare themselves the extra paperwork. He second-guesses his motivation. He has nightmares.

Cantú’s writing is engaging and straightforward. At times, it is achingly lyrical, such as when Cantú, now at a desk job with the patrol, sees a falcon peering into a security camera he monitors and reads pressing questions in the bird’s curious look: “What cowardice has caused you to retreat from the ragged heart of the desert? Why not return to the desert’s smoldering edges, why not inhabit the quiet chaos churning in your mind?”

Cantú’s portrait of Mexico as the backdrop for border crossings comes to feel too unrelentingly bleak — all mutilated bodies, crushing poverty and crumbling towns. Some readers might lose patience with the author’s conflicted feelings about a job he stuck with for years. But if they are interested in life on a border that has recently occupied an outsize role in policy debates, they will learn a lot.

The final third of Cantú’s book chronicles a reversal for the author: In the years after leaving the Border Patrol, he befriends a Mexican maintenance worker. When the man travels to Mexico to see his dying mother and can’t cross back, Cantú takes on a wholly different kind of journey of discovery, from an immigration courtroom to a detention center to a Mexican border town where his friend reflects on his unapologetic determination to return.

Mila Koumpilova covers issues of immigration for the Star Tribune.

The Line Becomes a River
By: Francisco Cant
Publisher: Riverhead, 250 pages, $26.