Dropping her 3 1/2-year-old daughter off at school is the most nerve-racking. 'She seems so young to be going to school. It is the first day in her entire life she has been out of our care.'
As for the Booker, which Random has picked up two years running, she says: 'I'm not sure if it's going to be a bumper year for Random House.'
Her fears prove correct: the list of six contains only one Random name, Ian McEwan for Black Dogs (Cape).
The Random House UK group embraces Jonathan Cape, Chatto & Windus, Bodley Head, Century, Hutchinson and Arrow paperbacks, plus a dozen smaller imprints.
When I call back, she is 'delighted' about McEwan, but 'shocked and disappointed' that two Random entrants, whom she refuses to name, did not make the final six.
Tall and lean under a mass of dark hair (she tries the diets she publishes), Ms Rebuck comes across as a forceful, dynamic figure. But for someone at the top of a profession notorious for bitchiness, she is strangely afraid of speaking her mind, at least in public.
She won't name the novels the Booker judges were wrong to omit; she starts to say that Robert Harris, author of the best-selling Fatherland, a Random book, will come to be ranked alongside Le Carre, but hastily retracts, saying: 'Dare I? Scrub that.' And she will not be drawn on the controversial Net Book Agreement.
She is happy to talk about her two infant daughters, but not about her husband, Philip Gould, the Labour Party political adviser.
Her reluctance is partly understandable. She took charge a year ago, when Random House Inc, the American parent and the biggest publisher on that side of the Atlantic, decided that the face of Anthony Cheetham, her boss and mentor, no longer fitted. In a swift and brutal execution, Mr Cheetham, the flamboyant doyen of British publishing, was dispatched, to be replaced by Ms Rebuck, a 39-year-old Sussex University graduate.
The knives were out. So Ms Rebuck has spent a lot of time since making sure the target is as small as possible. It undoubtedly helps that her husband, who also advises foreign governments, is well versed in the art of PR, of ensuring that, as in the case of Neil Kinnock, traps are avoided and awkward questions go unanswered.
Then there is Random itself to consider. In an industry where the image still endures of publishers working amid civilised surroundings in Bloomsbury, meeting their authors for a convivial glass or two and putting creativity before profit, Random is seen as something of a monster.
It operates out of a glass-and-steel office in the unfashionable and ungenteel Vauxhall Bridge Road. Brash and vulgar, say its critics, it cares more for sales than creativity. Art is merely a commodity - another way of making money, like oil and steel.
To these critics, the ascendancy of Ms Rebuck - a businesswoman who netted pounds 1m personally from the sale of Cheetham's Century to Random (and spent part of the money on a Majorcan farmhouse) - was further proof. She was known as a power player, a lover of sales conferences and big numbers. Her claim to fame was making millions from the likes of Anton Mosimann, the chef; Kaffe Fassett, the knitting writer; the Daleswoman Hannah Hauxwell, and Callanetics - rather than supplying a cerebral read.
A year later the critics are on the defensive. There have been few outward signs of expected upheaval and discontent. The Ruth Rendells, Kingsley Amises and Anita Brookners have not upped and left. The one big internal shake-up she has engineered, the merger of the hardback and paperback sales forces, seems to have gone smoothly. Random still produces 2,000 titles a year and goes out of its way to cosset the smaller independent bookseller as well as the leading chains.
And while the Bloomsbury set is struggling in the worst recession publishing has known, Random at least appears to be prospering. Latest profit figures are not forthcoming - as a private company Random is not obliged to publish them - but Ms Rebuck says the last year has been 'good for prizes' and for having 'a bumper crop on the bestseller list'.
And there is Robert Harris. Time and again in the conversation, Ms Rebuck returns to the subject of Mr Harris and his book.
Although a proven non-fiction author, with Selling Hitler and the biography of Bernard Ingham, Mr Harris wanted to become a serious novelist, so Random, in the shape of Hutchinson, and Ms Rebuck taking a personal interest, nurtured and encouraged him.
The result is a runaway worldwide triumph that has sold more than 250,000 copies in hardback alone. In the UK, she says with evident pride, 'everyone thought the hardback fiction market was dead. But Fatherland, a skilfully crafted thriller based on a world where Hitler is still alive and Germany won the war, was at No 1 from the day it was published, and has sold over 30,000 copies.'
It is sold, she adds with a smile, 'at the full pounds 14.99 price'.
Harris, she maintains, is proof of everything that is good about Random - and, although she does not say it, about Ms Rebuck.
'Authors want three things from their publishers. They want sales, and to be plugged into a great sales machine, worldwide. But they also want cosiness and intimacy, to enjoy a close relationship with a small number of people. And they want to be edited with care. This is the challenge publishers face. And only a large organisation like ours can rise to it.'
As soon as she had read Mr Harris's draft, she faxed Harry Evans, the Random chief in New York, to say that the US parent should take it on. Ms Rebuck and Mr Evans, the Times ex- editor, talked about it, and Evans now has a US bestseller of which he, too, is immensely proud.
The book's transatlantic success, she says, just shows what Random is capable of. While Mr Harris was known in Britain - albeit for non-fiction - he was a total unknown in America. But the right promotion and sales pitch took him to the top of the New York Times list.
She has now signed up Mr Harris for a further three novels. 'Fatherland is an important book,' she says. 'After Deighton, Forsyth and Le Carre, everyone is looking for the next generation. Robert is the first to come through.' She pauses and reflects. 'That is what is exciting and interesting about this job.'
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