'It's a scary time. I'm not young - I'm 66 - and I don't know what's coming next," confesses Ruth Leon, as if answering the question written on the cover of her memoir.

Called But What Comes After?, it describes Leon's relationship with Sheridan Morley -the biographer, theatre critic and broadcaster to whom she was married until his death in 2007.

The book is no sentimental look back at marital bliss. Its main focus is on the final decade of Morley's life, when he was whittled down by a cocktail of illnesses, including suicidal depression, strokes and diabetes, until the man Leon married was hardly recognisable and the woman Morley married was forced to take on the role of nursemaid.

"I wanted the book to be useful", says Leon. "I wanted people who find themselves in this situation to have something that said: 'You're not alone'."

The relationship began platonically in 1960 when a 16-year-old, petite "Jewish princess from Regent's Park" visited a friend at Oxford University and found herself in the company of a charmer with "broad shoulders, slim hips and long legs". They soon found they both loved theatre. Sheridan's already considerable claim to fame was that his father was the film star Robert Morley, an actor who had cornered the market in roles demanding English pomposity.

Decades later, by which time Sheridan was the best known arts journalist in the country and Leon had carved a career in America and here as a critic and television producer, the two lifelong friends decided to get divorced from their partners and married to each other.

To some it may have appeared an unlikely union. The son of the quintessentially English Robert Morley getting hitched to the Jewish daughter of a shmutter merchant? Leon does not see it that way. Her Jewishness was cultural and never impinged on her Englishness. And anyway, the first conversation she had with Sheridan's famous father revealed that all was not quite as it seemed. "Almost the first thing he said to me was: 'You know that I'm Jewish of course'." recalls Leon. "And I sort of looked at him and thought: 'I can't imagine anybody less Jewish than you are'. And he said: 'Oh yes. My mother'.

It turned out that his mother had been born to a German-Jewish family and he was in fact by birth Jewish."

Sitting in the living room of her modern, Thames-side house in Battersea, Leon is surrounded by the trappings of a life spent in and writing about the arts. There are lots of books, of course, while the lid of the grand piano is covered with framed photographs of family, including her Polish shtetl-born father, Sam, who ran a clothes shop in Marylebone Road with Leon's mother, Rose.

Sitting on the sofa next to Leon is an open laptop, and the purring Byline. The cat's name has taken on an ironic twist since Leon's book revealed that when Morley was too ill to write his review copy, she would do it for him under his byline, even when Morley was unable to see the show.

To Leon's frustration the revelation has received more attention than her descriptions of Morley's descent into mental and physical breakdown. (It was even misreported that she had not seen one of the shows. )

"Look, it was a family business," she explains about the byline issue. "I came from a family business and when my father was ill, my mother took over the shop."

Fair enough. What wife wouldn't help her unwell husband if she could? On the other hand, the shop analogy only goes so far. Sher-idan's byline did not say "Morley and Co".

"That's true. But I would do it again. I would do it again in a minute," she says defiantly.

To say the least, Leon had a lot on her plate at the time. She had two jobs plus there was her husband's deteriorating health to cope with. The depression was not new, but after the stroke in 2001 the drugs that had kept it at bay no longer worked. The doctors were baffled. All hope of saving Morley from a suicidal condition that prevented him from getting out of bed, let alone going to the theatre and broadcasting his radio show, rested on a hugely risky operation in which a sort of "pacemaker" would be implanted into his brain. Morley was so ill that it was left to Leon to decide to go ahead.

Around this time, Leon kept going to first nights. Morley was getting a reputation for nodding off during shows, but there was little sign of what she was having to deal with at home - apart from, that is, the night he keeled over onto the hard floor as the couple left the Barbican Theatre.

I was at the Barbican that night and arrived on the scene to find Sheridan only semi-conscious. But the image that stays is of the relatively tiny Leon kneeling next to this big, helpless man. Yet despite being surrounded by people wanting to help, she cut a lonely figure too.

"It was very lonely all the time, partly because of the guilt. And it wasn't just because I'm Jewish that I felt guilty, but because I wasn't a good enough carer." She says she lacks what she calls 'the Mother Teresa gene'. But even though during his more lucid moments Morley often recognised that he had become impossible to live with and expressed his fear that one day she would leave him, the thought never occurred to her. Never?

"Never. Murdering him, yes," she jokes, "leaving him, no. But I was never meant to take care of people. I don't have children. And there is a reason. My life is so full, there is so much in it, I didn't think I would make a very good mother. I was fiftysomething when Sheridan had his stroke. And then I suddenly had a baby to look after - a very difficult, recalcitrant, 16-stone baby."

There may, she says, be on unintended result of the memoir. It might make people think of her differently. "I think if anyone thought I was invulnerable…" she pauses, searching for the right words. "My mother used to say: 'Ruth is the shtarker - the strong one'. Well, sometimes I'm not. And I think if anyone assumed I could cope with anything, I've certainly changed that with this book."

Perhaps. Then again, many people would have crumbled at keeping two high-pressure jobs going while their spouse and soul-mate disintegrated. Leon never stopped working or pushing her husband's doctors for new solutions. The memory I have of her is being back in the theatre reviewing pretty quickly after Sheridan's death. "It was two days," she says, candid as ever. "Was I going to sit at home and weep?"

I offer some amateur psychology for the quick return. Morley had been ill for years before he died and during much of this time his depression had deprived Leon of the man she fell in love with. By the time he died, she had already done a lot of mourning. It turns out there might be some truth to this. After Morley's death Leon visited his psychotherapist.

"I said: 'I'm not doing this right. I see other people who have lost their husbands and they are puddles on the floor. I'm not. What's wrong me?' And he said: 'You have been mourning Sheridan for four years, what do you expect?'"

He is probably right. Though so too is Leon's late mother, and the truth probably lies somewhere between the two.

'But What Comes After?' is published by Constable at £16.99