Comedian Bill Cosby Image Credit: John Minchillo/Invision/AP

The author of a new Bill Cosby biography is apologising for not pursuing allegations that the comedian had drugged and sexually assaulted numerous women.

Mark Whitaker, whose Cosby: His Life and Times was published in September, tweeted on November 24 that he was wrong not to “aggressively” look into the charges and promised to address them “at the appropriate time”.

“If true, the stories are shocking and horrible,” wrote Whitaker, whose account was confirmed by his publisher, Simon & Schuster.

Reports about Cosby have been public knowledge for years, but resurfaced in October after a video of comedian Hannibal Buress calling Cosby a rapist went viral. With several additional women accusing Cosby of assaulting them, projects on NBC and Netflix have been cancelled, and TV Land decided not to air reruns of The Cosby Show.

Cosby, 77, has issued a statement denying the accusations.

Whitaker’s book has been widely criticised for idealising Cosby, until recently one of the country’s most beloved entertainers, although some critics praised it upon release. Amazon.com included Cosby among its best books of September and top 100 biographies and memoirs of 2014. The book includes blurbs from David Letterman, Jerry Seinfeld and Mary Tyler Moore.

“If I was America’s ‘sweetheart’ — turning the world on with a smile — then Bill Cosby was and still is our ‘best man,’” Moore wrote.

“A revealing, honest look at my favourite comedian,” reads a blurb from Billy Crystal.

According to Nielsen BookScan, which tracks around 80 per cent of print sales, Cosby has sold around 6,000 copies. It’s a small number for a well-publicised book, but not unusual in an increasingly tight market for hardcover non-fiction, which has been hurt by the decline of physical stores. As of midday on November 25, Cosby ranked No. 13,711 on Amazon.

Cosby cooperated with Whitaker, agreeing to be interviewed. But, according to Simon & Schuster, Cosby had no control over the book’s contents.