When Barbara “B.” Smith announced she was closing her eponymous restaurants in Sag Harbor and Washington, D.C., in 2013, the public didn’t know of any problem beyond the vagaries of the food business. But Dan Gasby, her husband of more than 20 years, did. Smith had become forgetful and temperamental, losing her curiosity and starting to hoard things, mostly clothes (unsurprising for a former fashion model and longtime paragon of style).

As doctors told them that year, Smith, then 64, had early-onset Alzheimer’s disease.

In a new memoir, Gasby and Smith, with the aid of Vanity Fair contributing editor Michael Shnayerson, chronicle Smith’s illness and its effects on them and their daughter, Dana. Told in short chapters from both their perspectives — most, for obvious reasons, from Gasby’s — “Before I Forget” is marked by strong emotions and often bleak honesty.

Gasby describes his wife’s “classic Alzheimer’s behavior”: becoming not just forgetful but also angry, lashing out, pushing her family away. He recounts painful insights: “Tell me words that start with the letter F,” a doctor asked early on, and Smith — who still seemed mostly lucid to her husband — could only come up with “Finger . . . food . . .” Asked what year she was born, she suggested 1942 — or maybe “ninety forty-nine.”

They watch the Oscars on television, and Smith cheers when Julianne Moore wins for her role as a woman with Alzheimer’s in “Still Alice.” And there’s the scary tale of the time Smith simply disappeared for 18 hours before friends found her in a diner, disheveled and disoriented after walking the streets all night. Smith writes that she doesn’t remember why she started wandering: “ I do remember it was the evening, so it didn’t seem . . . out of place. And I remember it felt good. . . . . I was taking care of myself, having an adventure. I know that’s not a really good thing now.”

These stories are interspersed with smart, informative essays on the state of Alzheimer’s research, various drugs, tips on caregivers and support groups, how the disease typically progresses and how there is increasing evidence that it disproportionately affects African-Americans. There are also flashbacks to pre-Alzheimer’s days, including when Gasby met Smith in 1987: “That first night I saw her, B. was wearing a fire-engine-red bustier and red hostess gown — and managing to look demure while doing it. Man, I remember thinking, it’s like they poured chocolate into the perfect mold.”

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But it’s the personal insights that are the most compelling.

Smith: “I don’t want to feel like this. I’ve always been an emotional person, but it’s different. I cry a lot. I don’t know why. . . . I want to be nice to my family, but sometimes I can’t be.”

Gasby: “Sometimes, just as I’m gripped by despair, a glimpse of the old B. shines through. . . . The fog can lift, and when it does, the intensity of those moments takes my breath away. We look at each other, and it’s like she’s in a rowboat at the end of a pier where I’m sitting, and we’re close enough to touch. Then the tide starts to pull the rowboat away, and again the fog rolls in, until the boat disappears.”