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The publication of Sir Terry Pratchett’s final novel six months after his death is “a celebration” of the best-selling writer, according to one of his closest friends.

Rob Wilkins, who was Sir Terry’s assistant, gave a reading from The Shepherd’s Crown at a midnight launch in Waterstones in Piccadilly.

Fans in fancy dress gathered at the store to get their hands on the book - the 41st and final instalment in the Discworld series which was written by Sir Terry before his death aged 66 in March, eight years after he was diagnosed with Alzheimer’s disease.

Mr Wilkins, 46, told the Standard: “This is Terry’s final novel and to me there can be nothing more important. This has got to be a celebration of him, all of his work culminating and giving him a right good send off.

“He was so modest that he would look at all of this and wonder what all the fuss was about. We are going to have a proper memorial next year. But this is it, the final Discworld novel.

“There will be spin-off projects and things like that going off into the future and also we have Narrativia, our film and TV company. We are actively out there developing material at the moment.

“Terry’s daughter Rhianna is actually working on the script right now for The We Free Men.”

Sir Terry’s illness eventually robbed him of the ability to use a keyboard and he dictated his last books with the help of Mr Wilkins and using computer software.

Mr Wilkins added: “I actually say in the back of the book it wasn’t finished as Terry would have liked. What I mean by that is he wrote the book like a giant literary jigsaw then pieced it all together at the end and kept polishing it until you couldn’t see the join between the various parts and once it was assembled then he would be happy. But he didn’t quite get to the point of assembling all of the bits when he died.

“Being at the coal face with all those ideas coming out – you would never know the direction he was going in. He would even throw me a curved ball.

“We were lucky with his Alzheimer’s, his variant meant we had more of Terry for a longer time. It was only the last few months that we started losing the Terry we loved. In January that was the start of the end and it was terribly upsetting.

“Terry wasn’t a sentimental man, we never had our final discussion or the final chat or anything like that. Every day was a new day and you just dealt with the problems his disease brought on a day to day basis.”

Among the gathered fans was Sarah Snell-Pym, 34, a writer and artist from Gloucester, who attended with her daughter Jean Pym, 9, and father Leonard Pym, 68.

She said: “Terry was working so hard towards the book. It would have been an extra heartache if he hadn’t managed to finish it.

“This is really important for me as it’s the first book launch my dad and I have both been healthy enough to go to together.

“Terry Pratchett is very important to us, he got dad through his heart attack and I was incredibly ill after pregnancy and whenever one of us ended up in hospital we’d read Discworld novels.

“It’s been a huge thing for us. It’s bittersweet because Terry’s not with us anymore but it’s so important.”

Louie Reynolds, 30, from Southampton, dressed as death for the event and said: “It’s very sad it’s the last one. It’s the last book but it’s a celebration as well for Terry Pratchett. I was devastated when he died. It was inevitable but you just think he’s going to go on and on and the books would keep coming. He kept writing until the end.

“He had a great heart. It’s the end of a legacy, a huge set of books that nearly everyone in the world has probably heard of in some way. His imagination is hugely inspirational to other authors out there, to children and readers out there.”