Ian McEwan is very much a literary rock star (witness the talks he gave at Stanford winter quarter), but one who hasn’t crossed the line into celebrity and ostentation, like Chuck Palahniuk and Jonathan Safran Foer. He writes good novels, his prose is perhaps the best in the English language (although Claire Messud in “The Emperor’s Children” might give him a run for his money) and he’s unafraid to write about the most difficult of topics, sex.

That topic remains the focus of his latest novel, “On Chesil Beach” — or novella, for it is a small-dimensioned book and barely 200 pages, and easily readable in one day. The first third of the book was already published as a short story in The New Yorker, and was excellent on its own. The question, however, is whether the last two thirds of the novella add anything to it.

McEwan is all about detail, and the premise of his story is mired in it. An English couple in 1962 (“a time when a conversation about sexual difficulties was plainly impossible”), just married and are to consummate their love.

The consummation proceeds awkwardly, as one would expect. It’s heinously awkward. McEwan’s omniscient narrator alternates between both character’s perspectives, and fills in much of the back story in alternate chapters. It comprises the most interesting parts of the novel — Edward’s desire to hide his “mentally damaged” mother from the world and his self-centered ambition as well as Florence’s introverted, singular focus on music and her emotionally distant relationship with her parents. Florence and Edward come from different social classes, but agree politically; he wants more physically, and despite her belief that she loves him (and the reader does come to believe that), she is repulsed by most physicality.

Their approaches to marriage and the wedding night itself seem to be shaped by their personal psychologies, the dynamics of their interaction and Florence’s sense of “duty.” At one point, Florence revealingly comments that she sometimes feels like a mother, sometimes a daughter, to Edward. He is missing a real mother figure, she has daddy problems. It works.

McEwan can only squeeze so much out the story, however, and it proceeds slowly, bordering on too slowly, at 203 pages. Much more could have been spent on the aftermath — divorce due to “nonconsummation;” Edward’s frequent romantic disappointments; Florence looking for him in the seat he once promised he would sit in at her new quartet’s premiere, several years afterward. It is hard to believe that after more than a year of intense emotional interaction, they would never speak again for the rest of their lives, much less try to find each other again. One wonders if perhaps focus could have been not on the aborted consummation itself, but rather its effect on their lives.

“On Chesil Beach” is too small, however and too limited to become McEwan’s defining novel. It is much less ambitious than most of McEwan’s previous novels. For a full-length work, it’s very focused on the two newlyweds — they’re the only characters, really — and rarely delves into larger social themes, as he did in his novel “Saturday.” “On Chesil Beach” might have worked better as a short story, even if it was broken up into two or three installments in The New Yorker. Written by a novelist with such prodigious and obvious talent, it is somewhat disappointing. Still, it’s a short book and worth reading, if only to savor the prose.