Hotshot literary agent Jonny Geller has done the unthinkable. Not only has he crossed to the other side to write his own book - he's put the word Jew on the front cover. Yes, But Is It Good for the Jews? is a beginner's guide to the science of Judology. Geller's book defines this mystical philosophy as "a cousin of mathematics, a sister to the talmudic art of gematriya - apportioning numerical value to individual letters in the Torah and thereby giving them mystical significance - a third cousin of Kabbala." For the first time the Judological Institute of Spiritual Mathematics, based in Cockfosters, Hertfordshire, England, is sharing its mathematical formula with the outside world. The following equation determines whether or not something is good for the Jews. Anti-Semitic potential (otherwise known as backlash) + impact on the world x the J-factor = tzures (trouble). Divide the result by seven, the mystical kabbalistic number. Apply this foolproof calculation to anything - from KY jelly to rhinoplasty - to weigh up its relative benefits vis- -vis the Jewish people. A round of applause for Google, the Eurovision and Scarlett Johansson, but steer clear of Desperate Housewives, Ikea and tattoos. The idea was born of a joke. Geller, 39, whose clients include Vikram Seth, Tracey Chevalier, Howard Jacobson and Hari Kunzru, says, "I was talking to a publisher about something quite obscure like high discount clauses in publishing. He said it was good for publishers and I said, 'Yes, but is it good for the Jews?' "He started answering seriously, so I told him it was a joke. He e-mailed me later to say it was a good idea for a book." One of the first entries Geller wrote was about Prince Harry. The third in the line to the British throne sparked a storm of controversy last year when he turned up at a fancy dress party in Nazi uniform. "While the prince prancing around as a Nazi was terrible and ignorant, it was two weeks before the 60th anniversary of the liberation of Auschwitz. This important event might have gone unnoticed, but because of Harry it made front page news. So, in a way that was good for the Jews," he says. This, according to Geller, is a prime example of how Jewish humor acts as a defense mechanism. But will everyone see the funny side? "I was never worried," says Geller, a father of three. "Everyone around me was. About two months before the book came out, my wife woke up in the middle of the night and said, 'Just cancel it. Don't do it.' "There was also a danger of people taking it out of context, that other people would say, 'Why do they think like that? Isn't it narrow-minded?' I would hope that after reading all the relentless jokes, people might get the point." As far as Geller is concerned, it's not so much the content, but the title, that is controversial. "The biggest problem for most people - Jewish or not - is having the word Jew on the front cover." He recalls pitching the book to a gathering of uncomfortable salespeople at Penguin, his British publisher. "I said, 'I feel there's an elephant in the room and that elephant is the word Jew.' I told them to find their inner Jew and shout Jew at me. They looked at me, did it, then all cracked up laughing. It certainly broke the ice." Originally intended for the US market, the book has received a markedly different reaction on either side of the Atlantic. "Most people I saw were aged 55 and over and they really just wanted a book of Jewish jokes," says Geller, who recently returned from a promotional book tour in the States. "A lot of them wanted to talk about serious stuff, like Bush and jihad. I think they took it all a bit literally." By coincidence, the book was released at the same time as the movie Borat, the alter-ego of Sacha Baron Cohen, Geller's client and friend from his school days. "In America everyone wanted to talk about Borat and whether or not he was anti-Semitic, but nobody has mentioned him to me here." Geller's formative years in suburban London provide much of the inspiration for the book. "My grandma used to sit asleep in front of the TV on a Friday night. The newsreader would say, 'And now to Israel,' and she would suddenly open her eyes and watch intently. The minute the newsreader said, 'Now back to domestic news,' she would fall asleep again." This tendency to watch out for one's own is not exclusive to Jews. "I think everybody shudders a bit when they see someone from their community doing something wrong, like going to prison," says Geller. After finishing school, Geller spent time in Israel in yeshiva and the Habonim youth movement's Shnat Sherut program. He returned to the UK to study English and French at university, and trained as an actor after graduating. But touring provincial theaters in Agatha Christie plays did not provide the big break he had hoped for. A premature mid-life crisis at 25 saw him abandon the theater for life as a door-to-door salesman - at which he was surprisingly good. Eventually a friend encouraged him to combine his love of books with his talent for selling and apply them in a different field. In 1993 he joined Curtis Brown, one of London's leading literary agencies, as an assistant. He soon made his mark, however, when he discovered unknown author Martyn Bedford and secured 250,000 for his book, Acts of Revision. Now managing director of Curtis Brown's books division, he has an A-list of clients. Geller has no plans for a sequel - but doesn't rule out the possibility of another book. "I think I'm much more understanding of the writing and publishing process now. It's great fun - I can see why writers become addicted to it."