A good writer can make any material sing. We hardly need another midlife crisis novel, marriage breakdown novel or sexual awakening novel, so it must be the singular ability of film-maker, artist and writer Miranda July – coming along to show everyone else how it’s done – that makes her new novel, All Fours, seem essential.

Of course, we know to expect the unexpected with July. Her previous (and debut) novel, The First Bad Man (2015), was perpetually surprising and subversive: first, funny, then strange and, finally, emotionally devastating. Her new novel manages to be a bit of all three, though less firmly divided than before. Now everyone is older and things aren’t so clear any more.

The unnamed narrator is, like July, an artist who’s had success in “several mediums”. She’s “busy … but I always have time to worry”, and she lives in California with her husband, Harris, and their child, Sam (whose gender is undeclared). At the start of her story, she’s 45 years old, which she reckons is the midpoint of her life. “Or if you thought of it as two lives, then I was at the very start of my second life.”

What will the second life consist of? Well, there’ll be plenty of time for jokes, and July’s are among the best in the business. She frets about getting older: when a man in his 80s winks at her, she wonders: “Was that how old a person had to be to think I was hot these days?” She feels sexually distanced from her husband. “Sometimes I could hear Harris’s dick whistling impatiently like a teakettle, at higher and higher pitches until I finally couldn’t take it and so I initiated.”

But beyond the quips, July has her eye on something richer and stranger, incorporating a stalled road trip, a sexless love affair, and a marriage taken apart and reassembled in a different form. Our woman heads cross-country to New York alone, but unknown to her family she never gets there. Instead, she stops at the nearby city of Monrovia, where she checks into a down-at-heel motel. (“Were the nylon curtains intentionally ecru or just dingy?”) There, while helping to renovate one of the rooms, she falls for a man named Davey whom she keeps bumping into, and falls for him, deeply and steeply.

What becomes clear on this voyage of self-discovery is that our narrator’s past is not past. Both her grandmother and her aunt died after throwing themselves out of a window – the same window – and she worries “that I was next in this matriarchal lineage”. But most of all, she has never recovered from the trauma of Sam’s birth, when they both suffered foetal-maternal haemorrhage, a catastrophic blood loss that is often fatal. She still scours the internet trying to understand what happened, but what is clear is that this “shared nightmare” is the strongest bond that she and Harris have.

July switches between modes in a way that allows comedy to amplify the sadness rather than undermine it. What comes next in the story it would be unfair to reveal, but it continues to balance that line between absurdity and emotional intensity. When she turned 45, the narrator’s existence seemed “perfectly still for a moment. Neither rising nor falling,” but now she is hurtling towards something. It feels as if July is hurtling too: the structure is looser than The First Bad Man, which is at times frustrating, but within one idiosyncratic story, this is a book of vast scope, taking in men and women, the mind and the body, and society and solitude.

By giving her narrator some of her own biographical details, July is playing with the reader’s expectations in a tradition that runs from Marguerite Duras through Christopher Isherwood to Rachel Cusk. But she’s funnier and more peculiar than any of them. Even the title fits: a phrase at once commonplace and odd, it becomes intrinsically funny when stripped of context. So when the narrator writes of one of her unusual actions, “it wasn’t a performance … nothing I did ever was. It was only ever the truth of the moment,” it sounds like character and author at one and the same time – and we believe her.

• All Fours by Miranda July is published by Canongate (£20). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply