When I was at school, children with Irish parents would often disappear to Ireland for the entire summer holidays. And when they got there, their cousins would greet them with "How long are you home?" This is a sweet, profound hospitality but it does imply that your primary place of residence is a shadowy exile and that the more abundant life is elsewhere.

This is how things are in the mind of Holly Hogan - a 14-year-old girl who lives in a care home at the beginning of Siobhan Dowd's second posthumous novel. When Holly's favourite care worker leaves for a job "north of the river", she is fostered out to a well-meaning middle class couple. Materially she is home and dry but Holly is sure that "home" should be more than this, and that if she can just get to Ireland she will find her lost mother and a better life. She borrows her foster mother's wig to make herself look older and sets off.

I would love to write this review without referring to the fact that Dowd died in 2007, aged 47, of breast cancer, not least because I'd love her not to be dead. But the wig won't let me. Holly's foster mother bought it to hide her baldness during chemotherapy for breast cancer. It's an extraordinarily powerful image - a girl on the cusp of adulthood, with this glamorous, sexualised memento mori on her head, strolling off into the valley of the shadow of death via the A40. Her route connects the place where Dowd ended her days - Oxford - with the place her people came from.

Holly is an engaging and convincing character, smart and hard enough to find her foster mother's weak spot, but naive enough to think she can walk to happiness. When she wears the wig, she calls herself by a different name (Solace) and this split in her identity empowers her to go to places and start conversations that she really, really shouldn't. All the way through the book, you have the sickening feeling that something terrible is going to happen to her. She's picked up by a much older boy in a nightclub. She gets a lift from a lonely pig-farmer. It sounds like a formula for the grimmest of grim lit. But - I don't want to give the end away - things take a surprising turn. Life, though it is fragile, is not grim at all. Although she is beset by danger on every side, Holly also encounters hospitality, and casual kindness. I don't think it's just the fact that we know that Dowd died tragically young that makes this story - with all its realistic detail, its iPods and its pay-as-you-go phones - float like a parable.

Stories are chains of consequence, one thing leads to another. But some of the most sublime stories end when an act of grace or love that means "it ain't necessarily so". Abraham doesn't have to sacrifice Isaac. The Green Knight has the right to decapitate Gawain but barely nicks him with his sword. The prodigal son thinks he has spent all his father's love but discovers that it is endless.

Dowd's glittering career fits more or less into the fearful gap between diagnosis and death. Here's a story about a journey which is equally fearful but which turns out to be worth it, thanks, as Holly says, to people who "did something to help me and asked for nothing back". This is a book which, despite its grim-sounding subject, turns out to be about the graces we gain when we just set out on the journey, and about good will which is the means of that grace and, which - to quote Cormac McCarthy - has the power "to heal men and bring them to safety long after all other resources are exhausted."

 Frank Cottrell Boyce's Cosmic is published by Macmillan. To order Solace of the Road for £9.99 with free UK p&p call Guardian book service on 0330 333 6846 or go to guardian.co.uk/bookshop