Like so many novels by Dick Francis, his latest puzzler revolves around a soft-spoken gentleman who once raced horses. It peeks inside a fascinating profession—in this case, mystery writing—to raise doubts about a man who may not be all he seems.

But this time the protagonist is Francis himself. According to Dick Francis: A Racing Life, an unauthorized biography by author Graham Lord published in England, the former jockey's much-loved mystery series may in fact have had two authors: Dick, 79, and his wife, Mary, 75.

Friends of Francis and many in publishing have long known that Mary researched and edited Dick's work. But Lord, 56, claims that in a 1980 conversation she implied that she also wrote substantial parts of the series. " 'It's much better for everyone to think that he writes them,' " Lord recalls her saying. " 'They're taut, masculine books that might otherwise lose their credibility.' "

Dick and Mary Francis—who, according to Lord, recently announced the crime series was nearing the finish line—have not responded to Lord's assertions, which were given wide currency in the British press. But their son Felix, 46, who is also Francis's manager, is both angered and saddened by the bio. "I fail to understand why Mr. Lord has chosen to write a book which will undoubtedly be hurtful to my parents," he says. He adds that neither his university-trained mother nor his father, who left school when he was 15, ever tried to hide their collaboration. "There was never a conspiracy to mislead or bamboozle. It was a partnership, a communion, but no trickery, no secret and no mystery."

Except to all those readers, of course.