Dear Jack: A Love Letter by Barbara Bates Conroy, Jackson Scott Conroy Klipspringer Press, 2018, 220 pages, $24.95.

“By the water we all feel at home,” writes Barbara Bates Conroy in her intensely moving book Dear Jack: A Love Letter, thinking back on the times she spent at Menemsha when her children were young. “The air is clean, the food is always fresh, and the people are my friends.”

The Vineyard is linked in her memories with deep rejuvenation — she calls it “the healing mecca” ­— but it also reminds her of the tragedy at the heart of her book: the loss of her son Jack at age 21, after years of betrayal and disappointment as he sank deeper and deeper into drug addiction. For the whole of his life, she wrote letters to him, and in the wake of his death, she kept writing them. Dear Jack is the result, a heartbreakingly raw and honest account of a mother’s love.

By the dates of the letters, readers can see that love at its brightest and purest, when Jack and his siblings are all young and healthy and happy. And they can watch that love darken, tatter and stagger. Jack is a likable, charismatic young man, a charmer, and as anyone familiar with drug addicts can predict, he quickly learns how to weaponize those qualities, constantly taking advantage of the goodwill or forbearance of his loved ones.

It makes for hard but mesmerizing reading, watching that degradation through the unfiltered immediacy of Conroy’s letters. In 2009 we find her writing, “I believe trust is the greatest virtue. I have lost trust in you. Not only do you lie but it happens constantly.”

There are ups and downs, cherished memories and moments when it seems like things might work out. But Jack continues to spiral downward, and no amount of counseling or rehab or tough love seems to slow the process for long.

“Dear Jack, I am picking up the bloody pieces of our family as once again your toxic behavior have left us destroyed,” one letter in 2013 opens. “I have been and still am desperate for your life to change, but it is out of my hands. There is zero trust, our relationship and that of your family and friends, shattered.”

Desperate, Conroy tries everything she can think of — she leaves notes, she urges rehab, she makes Jack sign contracts to follow rules — but none of it works. He steals money, he steals jewelry, he falls afoul of the law at every turn, he lies constantly.

“He would grab money from my wallet then stare me right in the eyes and tell me it wasn’t him,” Conroy writes. “He lied about everything, even things that didn’t matter.”

The book’s narrative alternates between letters addressed to Jack and confessional chapters of candid autobiography. In gradual shadings of nuance and repetition, readers steadily learn more about both Conroy and Jack, but the story keeps returning to that central worst moment, the moment in 2016 when Conroy learns her son is dead, the moment things went “from normal to incomprehensible.”

That moment opens the book — readers are never in any doubt about whether or not they can expect a happy ending — and that same harsh note is sounded at the end. “Addiction is a disease and the stakes are high,” Conroy writes at the close of her book. “Opioid and heroin overdoses are increasing at alarming rates. These drugs are killing a generation of kids. They killed mine.”

And yet, Dear Jack is ultimately a hopeful book. It is a story crowded all around with rage, despair, disappointment and grief, but it’s also a celebration of love, a treasuring of happy memories, and a chronicle of healing from the worst pain imaginable. It’s a somber but very brave work of remembrance.

Barbara Bates Conroy will be speaking about her book and experience at Salte in Edgartown on August 14 at 11:30 a.m. and at the Chilmark Library on August 29 at 5 p.m.