The Infatuations, By Javier Marías, trans. Margaret Jull Costa

Javier Marías does lots of things novelists aren't meant to do. He tells, not shows. His sentences are long and flowery, with sub-clauses hanging off sub-clause like chains of tropical flowers. There is little action. He's prone to lengthy philosophical detours. Yet he constructs some of Europe's finest novels, and not just academics or highbrow critics celebrate him: he is popular, selling big in dozens of countries.

Marías published his first novel in Spain in 1971, aged 19. In the next two decades, he published four others, translated several from the English (winning a prize for Tristram Shandy – a clear influence) and taught in Spain, the US and at Oxford. Then, with the Oxford-set All Souls in 1989, he found his voice as a major novelist. It is the voice of a first-person narrator who observes intently, all the time doubting and speculating. His narrators' elliptical meditations are rich in erudition and shifts of emotion, which can suddenly change pace into tense action scenes. Proust comes to mind as his stylistic forerunner.

All Souls was followed by three novels in the 1990s, including A Heart So White (winner of the 1997 IMPAC award). After 2000, the 1500-page trilogy Your Face Tomorrow raised his already substantial reputation. Marías reverted to the Oxford narrator of All Souls, but the themes darkened, embracing a public world of secret agencies spying and killing on behalf of the British state.

Marías expresses a left-wing view, in his books (implicitly) and in actions. In 2012 he refused Spain's National Fiction Prize for The Infatuations. He wanted to maintain independence from the state; he complained too that his father, philosopher Julián Marías, a victim of the Franco dictatorship, never received recognition.

The Infatuations is confined to no more than half-a-dozen characters and settings in his native Madrid: a quiet street where a man is killed, a couple of flats, a cafeteria. Its air is claustrophobic. Overheard conversations are only half-understood.

For the first time, Marías sets himself the challenge of a female narrator, María Dolz, a publishing employee. She works well, but is little different from his other narrators: an observer, something of a voyeur. At breakfast, she watches a handsome, youngish couple in love. Marías readers know that such routine happiness cannot last and suddenly the "Perfect Couple, as she dubs them, is no longer seen at the café. Later, she sees in an old newspaper a photo of a man lying in the street: the husband, killed in a knife assault. The plot, the inquiry into what happened and why, is set. As usual, Marías makes unlikely situations feel plausible and creates a sense of sinister danger.

Marías barely suffers from the lack of authorial omniscience that the first-person entails. If he does want to tell you more than the narrator can see or overhear, his characters deliver a long monologue. The first person also enables Marías to indulge his pessimistic (and probably realistic) theme that other people's lives are unknowable. The most delightful person may be a murderer. Can we ever know what our lover is thinking?

The book's other themes are the fragility of normal life, whether love has any meaning and how easy it is to betray friends or lovers. Marías's method is to set up parallel relationships, and explore them. María and Javier, another Marías charmer whose morals do not match up to his handsome face, are mismatched. She is infatuated - or "in love" as the Spanish title Los enamoramientos suggests - while he is infatuated with Luisa. Though she's no fool, she acts like a fool. Is this love? Or diseased infatuation? Or is love merely infatuation?

It is pleasing to see Marías's fine translator, Margaret Jull Costa, given as much space inside the front flap as Marías himself. You do not notice her presence: when the translator vanishes, it means the translation's good. The Infatuations is not Marías's greatest novel: the tale is slighter than in others and the set-piece tours de force not so exciting. There is less humour, too, despite several fine scenes. The novel is pleasurable in its rhythm and in the voice, with its insights and doubts. Few writers catch so well the inner rhythms of a - neurotic - person's mind.