Blake Morrison is best known for the defining memoir he wrote about his father. His new book is about his Irish mother, Agnes O'Shea, one in a family of 20 children. Is it possible to write the same book twice? Rosita Boland reports.

There are two literary genres that expanded with astonishing speed in the 1980s and 1990s as they became fashionable and popular.

In the 1980s, everyone was writing travel books. It seemed to be the fit-all-sizes genre; a bottomless hole into which one could pour anything and everything. Not many of these writers had anything interesting to say, so their books always aimed for showy effect rather than substance.

The other genre that expanded to bursting point was memoir; nothing was deemed untouchable - even incest between father and daughter, as in Kathryn Harrison's The Kiss.

Given all this, the writer Blake Morrison is both fortunate and unfortunate, since he wrote one of the defining memoirs of the 1990s; a decade which also saw so many terrible memoirs published.

Despite a well-respected reputation as poet, literary critic, and literary editor for both the Observer and the Independent on Sunday, it was not until he published his memoir, And When Did You Last See Your Father, in 1993, that his name became generally well-known. His book was that rare thing: a beautifully written memoir that transcended the very personal story of his father's death to become a thoughtful and moving meditation both on grief and death in general.

Since then, Morrison has left his job as a literary editor to focus full-time on his own work. He has co-edited anthologies, published his selected poems and written a controversial non-fiction book, As If, on the Jamie Bulger case. He has also continued to contribute literary criticism to newspapers. Two years ago, he published his first novel, The Justification of Johann Gutenberg. Yet the book he remains associated with is the memoir about this father.

Now, nine years after writing about one deceased parent, he has written a second memoir about the other deceased parent, his Irish-born mother Agnes O'Shea: Things My Mother Never Told Me.

Sitting opposite me in the foyer of a Dublin hotel, he perches on the edge of the sofa, waiting for the obvious questions with an air of slightly nervous resignation.

"Yes, I was surprised at the reaction to the book about my father. I had written it quite quickly. I think what people responded to was the exploration of grief." Why does he think the memoir genre became so popular? "I think people used to look to fiction for truth, but there is always invention in a novel. One of the things memoir did was to give true stories back to people."

By the time of her death, his mother had more or less edited out of her life her early years in Ireland. She left Ireland after training as a doctor in Dublin and worked in various British hospitals during the war, where she met Arthur Morrison, her husband-to-be, and set about shedding her Irish identity.

She changed her Christian name to Kim; the name by which her son always assumed she had been christened. She even let her husband talk her into giving up Catholicism, which had been such a part of her life. Although several members of her family remained in Ireland, around Killorglin, Co Kerry, where she was born, she only visited once with her children, when Blake Morrison was five.

She actively discouraged questions about her roots, and although Blake Morrison tried to formally interview her some years before her death, she evaded his questions, simply saying of her childhood, "It's so boring". Until her death, he had not known she was the 19th child of 20; many of whom had not survived infancy.

"I had the impulse to write about her life," he says. "Fair's fair. Why shouldn't she have a book about her too?" It's a rhetorical question.

Early on in the new book, Morrison quotes Dylan Thomas: "After the first death there is no other". Is it possible to write the same book twice?

"The first book was more about grief than anything else," Morrison says. (In Things My Mother Never Told Me, he admits that when his father died, he "wrote a memoir of our relationship as therapy".) "The second is the story of someone who is a bit of an enigma."

What's uncomfortable for the reader is the knowledge that, just before And When Did You Last See Your Father? was published, Agnes O'Shea told her daughter Gill (Blake's only sibling): "I could top myself because of that bloody book".

It's uneasy, complicit knowledge to have as a reader: the knowledge that this immensely private woman is now herself the subject of a book; one which acts as a companion piece to a book which had upset her so. And When Did You Last See Your Father? for instance, recorded the 10-year affair Blake Morrison's father had had with a woman known in their family as "Aunt Beatty".

The core of the new book is the scores of love letters which Arthur and Agnes wrote to each other before they married. Blake Morrison knew of the letters' existence, tied up in bundles. He moved them round the house as his mother got more ill, hiding them in a wardrobe, afraid she would throw them out. Shortly before her death, he took them all away from the house without telling her. He did not open them until some months after her death, but once he did, he felt he had the key to a new book.

"I had an instinct there might be something interesting in the letters," he says. He also admits that his mother would probably not have liked the book about her, which has her photograph on the cover.

"I think this book is better, it's more thoughtful," he says. But however one dresses it up, this book is not different enough from the first to make it anything as strong as that first book. Many of the letters make dull reading, and it is the letters that weave the narrative together.

Agnes O'Shea, mother of two, wife and doctor, was a private woman and her story is really not that interesting, even when in the hands of such a fine non-fiction writer as Blake Morrison. She assimilated into married life in Yorkshire and became in every way the less assertive part of that marriage. Although she loved her work as a doctor, and would have happily gone on working longer, her husband insisted they both retire together.

Hard for a reader is the knowledge that despite the many love letters printed here, the relationship they created together did not maintain its initial promise. What did Agnes feel about this? As Blake Morrison writes, there was no-one else before her husband, no-one during and no-one afterwards. But we never know what she really thought or felt about the way her life turned out; like Agnes, the book seems to fade from sight, even while being read.

There have not been many reviews of the book yet, but come they will, and surely nobody knows more about reviews than an author and former literary editor of a broadsheet newspaper. How does he handle reviews?

"With difficulty," he says, with straightforward honesty. "I never believe people who say they don't read reviews." He then quotes one or two reviews he's received which he didn't agree with, and, unintentionally, makes it clear that he does take reviews personally. There is nothing more personal than writing about your parents, so he is well aware of the flak that may lie ahead.

"This is the story I know best, so in some ways it is the most important," he admits. Then he grins across wryly, knowing he is showing his cards and not caring: "I'd love to write a novel that could affect people - like the memoir about my father did" .

Things My Mother Never Told Me, by Blake Morrison is published by Chatto & Windus at £16.99 sterling