When Marian Keyes announced she had written a sequel to her 1997 smash hit novel, Rachel’s Holiday, social media went into overdrive. Rachel’s Holiday – which sold more than 1.5m copies and spawned generations of devotees to Keyes’s writing – was a cultural phenomenon, following protagonist Rachel Walsh as she struggled to come to terms with drug and alcohol addiction during a spell in a Dublin rehab clinic, the Cloisters.

Again, Rachel picks up the story 20 years later, and Rachel is now an addiction counsellor at the clinic that once saved her. Where Rachel’s Holiday concluded with her estranged boyfriend, Luke Costello, declaring his love for her, here we learn that Rachel and Luke have now been acrimoniously divorced for six years, and have not spoken since. Rachel is dating Quin, “an adrenaline junkie, his particular poison being rock-climbing”, but in spite of his adoration of her, she is reluctant to commit fully. When Luke returns to Ireland for his mother’s funeral and he and Rachel meet up again, the truth about their failed relationship forces her to reposition the prism through which she had perceived the dissolution of their marriage.

Keyes, as ever, is dealing with a plethora of emotionally and psychologically knotty issues: as well as the driving narrative of addiction and recovery, Keyes tackles depression, sexual abuse, regret and grief. And yet typically she manages such a lightness of touch, effortlessly segueing from tragedy to comedy. There is chaos and humour in the life of the Walsh family, as when Rachel and her sisters prepare a “surprise” party for her mother’s 80th birthday.

Rachel’s ageing mum – a highlight in a novel replete with beautifully well-rounded secondary characters – issues strict instructions:

“Make them practise yelling, ‘SURPRISE!’… My sisters, but especially Imelda and Philomena, won’t want to, and some of the cousins are right bitches too, but tell them there’ll be no goody bag for them if they don’t.”

Grief also runs through the novel – both Rachel’s own and that of her patients at the Cloisters – and it is here that Keyes’s skill at characterisation really shines: each patient is so vividly depicted that the chapters dealing with their therapy meetings are like exquisite short stories; the mother who’s lost a daughter, the teenager devastated by his alcoholic father, the abandoned son.

But ultimately this is Rachel’s story, and as it unfolds, Keyes deftly draws parallels between her recovery and that of her patients, forcing her to reassess the progress she’s made and to confront painful truths about herself and her past.

If there’s a core theme in Again, Rachel, it’s that of forgiveness: forgiveness of oneself for human failings. Forgiveness for loved ones who don’t always behave in ways we might wish. Forgiveness for the adversities – seemingly perverse in their cruelty – that life sometimes throws at us. Again, Rachel has all of Keyes’s trademark wit, humour and whip-smart dialogue, but it’s also a novel teeming with compassion and redemption.

• Again, Rachel by Marian Keyes is published by Michael Joseph (£20). To support the Guardian and Observer order your copy at guardianbookshop.com. Delivery charges may apply