Fans of Howard Jacobson might be alarmed to discover that the main character in his latest novel is a Gentile. As it turns out, though, they needn’t worry. Julian Treslove may not be Jewish, but in most other respects he’s a typical Jacobson protagonist: a middle-aged man much given to tears, self-interrogation, a sense of imminent doom, falling heavily in love and regarding his male friends as his male rivals. Above all, he’s obsessed with Jews and Jewishness.

The Finkler Question (longlisted for this year's Man Booker prize) is full of wit, warmth, intelligence, human feeling and understanding. It is also beautifully written with that sophisticated and near invisible skill of the authentic writer. Technically the characterisation is impeccable, the prose a subtle delight, the word selection everywhere perfect, the phrase-making fresh and arresting without self-consciousness. Indeed, there's so much that is first rate in the manner of Jacobson's delivery that I could write all day on his deployment of language without once mentioning what the book is about. A single line describing the hero's father will have to do: "a man who stood so straight that he created a kind of architectural silence around himself".

The Finkler Question is very funny, utterly original, and addresses a topic of contemporary fascination. That is to say, it is about the anguish of middle-aged men, it consists of a series of loosely arranged episodes rich in argument and incident, and it examines how Jews now interrogate their relations with Israel.

It puts in play a gentile fascinated by Jews, and his two Jewish friends, one a Zionist comfortable in London, and the other an anti-Zionist comfortable in his outrage. They engage with each other in sometimes moving, sometimes bathetic ways, making their own journeys of self-understanding while they exasperatedly strive to educate each other.

The anti-Zionist Jew is called Finkler, hence the title of the novel. The "question" of "Finkler" is today's version of the "Jewish question". At the end of the 19th century, Jews asked of themselves, and were asked, "What is the future of the Jewish people?" At the end of the 20th century, this question had been reformulated as "What is the future of the Jewish state?" In Jacobson's book, Finkler dwells among those miscellaneous Jews who answer the question in versions of condemnation of Israel, Zionism, and Judaism.

The Finkler Question is a terrifying and ambitious novel, full of dangerous shallows and dark, deep water. It takes in the mysteries of male friendship, the relentlessness of grief and the lure of emotional parasitism. In its insistent interrogation of Jewishness – from the exploration of the relationship between the perpetrators of violence and hatred and their victims, to the idea of the individual at once in opposition to and in love with his or her culture – it is by turns breezily open and thought-provokingly opaque, and consistently wrong-foots the reader. For Treslove, the committed shape-shifter with little really at stake, such demands unsurprisingly prove rather too much. "Would he ever get to the bottom," he wonders, "of the things Finklers did and didn't do?"