London (Agence France-Presse) - Fans of late British author Terry Pratchett, many in science-fantasy costumes, queued outside bookshops overnight as the final instalment in the Discworld series, The Shepherd's Crown, went on sale Thursday.

Waterstones in central London was one of the shops to open on the stroke of midnight and the author's friend Rob Wilkins read an extract of the book to 200 ticket-holders.

Pratchett, who sold more than 85 million books worldwide, died in March aged 66 after a long battle with Alzheimer's disease. His Discworld novels are some of the best-selling works in English fiction around the world.

The series is set on a flat world balanced on the back of four elephants which themselves stand on the shell of a giant turtle.

Pratchett wrote the first book in the series, The Colour of Magic, in the late 1960s although it was not published until 1983.

The Shepherd's Crown, the 41st book in the saga, was completed last summer before he succumbed to the final stages of his disease. The book received five-star reviews in Thursday's newspapers.

"This isn't just a great Discworld book, it's extraordinary; a proper send-off for Pratchett and this mammoth series," wrote the Daily Telegraph's Kat Brown.

She added: "In this, his 41st Discworld novel, now his last, Pratchett gets his house in order beautifully."

"With his final Discworld tale, the late author continues his move away from pure fantasy and into moral and social exploration," wrote Guardian reviewer A.S. Byatt.

"We shall miss him," she added of Pratchett.