Alexander McCall Smith has written more than 50 books

The best-selling writer won the annual literary award at a Booksellers Association ceremony on Tuesday.

McCall Smith has written over 50 books, beginning the detective series in 1999.

The association honours authors who make the biggest impact on their business, and comedian Ricky Gervais spoke at the Brighton ceremony.

McCall Smith beat Harry Potter author JK Rowling, Iain Banks and Jacqueline Wilson to win the 2004 title.

Botswana

Born in Zimbabwe in 1948, McCall Smith was educated there before moving to Scotland to study.

He is a professor of medical law in Edinburgh, an international authority on genetics and an advisor to the United Nations' Unesco body and the government on bioethics.

His books range from specialist titles such as Forensic Aspects of Sleep to The Criminal Law of Botswana and children's novel The Perfect Hamburger.

There have been five instalments of the Detective Agency book series

With five instalments so far, the Number One Ladies' Detective Agency series features Precious Ramotswe, the founder of Botswana's first and only detective agency run by women.

McCall Smith said he was keen to show the "small, everyday Botswana that the tourist doesn't get to see".

The first self-titled Detective Agency book received two Booker Judges' Special Recommendations, and Oscar-winning director Anthony Minghella has bought the film rights to the Number One Ladies' Detective Agency.