How does one assess a marriage of 42 years? Which joys and grievances sum up a life together, and what questions linger? And how reliable are those recurring memories, anyway?

These are among the questions posed by Lily Tuck’s fearless and absorbing new novel, “I Married You For Happiness.” Tuck, a part-time Maine resident who won the National Book Award for her novel “The News From Paraguay,” forgoes conventional narrative here in favor of a bold, non-linear approach. She portrays the marriage of Nina and Philip in a barrage of flashbacks on the night that Philip suddenly dies.

That’s the novel’s brave conceit: For 200 or so pages, we barely leave their bedroom, which becomes a wheelhouse of sorts for newly widowed Nina, unexpectedly at sea. From the show-stopping first line — “His hand is growing cold; still she holds it” — readers journey to the core of their full and complicated marriage.

Philip is a mathematician — cheerful, charming and self-assured, forever casting things in a theoretical light. In class, he instructs students about probability, animating the topic with examples from real life. Mendel’s law of heredity becomes a discussion of hair color; binomial probability turns into a game of heads and tails. Probabilities, however abstruse, comfort Philip; they are his default mode. It’s the sort of analytical thinking that often rankles his wife, but which she now draws upon, as if to evoke him.

Nina is a painter, pragmatic and skeptical. “Her theory of the universe is that there is no theory,” Tuck writes. At times, jealousy consumes her. One day, Nina turns up at Philip’s office and overhears him on the phone exalting a woman named Isabelle. Upon inquiry, she finds that Isabelle is the name of a software program, “a generic theorem prover.”

The story moves from place to place as Nina glimpses the span of their marriage — their initial meeting in Paris; their home in Boston; vacations on Martha’s Vineyard, St. Martin, Guadeloupe.

The good life is marred mainly by Nina’s angst and a series of incidents that would come to haunt her. Among them is the loss of Philip’s wedding ring on a sailing trip and the car accident, years earlier, that Philip survived, but his young female passenger did not. These images play like themes and variations throughout the book, their details recast differently with each re-telling.

As Nina sits next to Philip’s body, she thinks of him alive, in the present, then quickly corrects herself. She wonders who will mow the lawn or fix the shutters. She wishes she could pray — Philip believed in God; she doesn’t — but isn’t sure what to pray for. In this limbo, reality and time are a muddle.

“She believes Philip loved her, but how can she be certain of this?” Tuck writes. “Knowledge is the goal of belief. But how can she justify her belief? Through logical proof?” Then, later: “Was he happy? Did she make him happy?”

This is that most interior of novels that lacks plot or action in the usual sense. Yet Tuck’s widow is virtually all over the map, moving backward, then forward, in time, circling around old secrets and obsessions. The wonder is that so much inner commotion reads as smooth as silk. What Tuck has captured so deftly is the essence of a bereaved wandering mind, with its detours and tangents, as it flits from one thing to the next. Stranger, still, is how lively the book is, given its circumstances. This is due, in large part, to Tuck’s rendering of the ever buoyant and cerebral Philip.

Suffice it to say, Tuck has taken the particulars of love and anguish, and parlayed them into the longest night of Nina’s life. The result is, by turns, intense, brutal and stunning.

Joan Silverman is a freelance writer who lives in Kennebunk.