Thousands of people will descend on Hay-on-Wye as stars including Goldie Hawn, Ian McEwan and Bob Geldof appear at the town's literary festival.
Around 80,000 visitors are expected at the festival
Rory Bremner, Terry Pratchett, Eoin Colfer and Stephen Fry are also on the bill for the 10-day event.
It is the 19th time the annual festival, which Bill Clinton attended in 2001, has been held.
It will include events and appearances from stars of cinema, comedy, music, politics and screenwriting.
Greg Dyke, Jon Snow, Dave Eggers, Philip Pullman, David Baddiel and Tony Benn are also due to appear. Eoin Colfer, the author of the popular Artemis Fowl series of children's books, said he was thrilled to be making his debut.
"It is my first time - it is really one of the top festivals in the world," he said.
"It is all book lovers together - everyone is united in their love of literature, and it is a pilgrimage for a lot of people. It is very relaxed and chatty and friendly."
He added that visitors could expect to be entertained at his talk.
"Because I tend to get such a wide range of people from 6-60, I tell stories, like ghost stories, and tell jokes," he said.
"I like to make it an experience the parents can enjoy as well."
Eoin Colfer said he enjoyed the atmosphere at festivals
Poet and writer Owen Sheers said a lot of the festival's character came from the town itself, as well as its literary side.
"I think you can't ignore the setting, it is an attraction in its own right," he said.
"But it is the interesting mix of events it has on offer - you are in a vibrant environment, with a lot of dialogue and questioning - you feel mentally active.
"It is getting bigger and more ambitious - I hope it will never lose its roots."
Festival organiser Peter Florence said they had already sold more tickets than last year.
"We have a new site, so we have much more space - it is much easier for the artists and for the public," he said.
He said the festival had taken on a life of its own.
"The wonderful thing is it has been adopted by the audience and the audience keeps growing - people keep bringing their friends and you see people who haven't met for 12 months - it is like Christmas without the family stuff," he said.
Mr Florence also said organising the festival was an ongoing process.
"Preparations for next year are already well under way," he said.
A cinema in the grounds of Hay Castle will put on a mixture of movies, previews and master classes during the festival.