The Hay Festival celebrates its 25th anniversary this year, bringing Nobel Prize-winning writers, philosophers, politicians and musicians to Hay-on-Wye to showcase their work and exchange ideas with an audience from all over Britain and beyond.

For writers and readers alike, Hay, with a population of just 1,500, has a special place in a crowded festival calendar.

Most of its events are held in marquees erected in fields normally grazed by sheep, and overlooked by rolling hills. This year's musical programme will be in a mediaeval castle.

Nobel laureates Doris Lessing and Orhan Pamuk are among previous visitors while this year's bill includes writers as diverse as Hilary Mantel, Edmund White and Salman Rushdie.

But for festival director Peter Florence, whose family was instrumental in setting it up, Hay remains essentially a "big clan gathering" and a deliberate attempt to spark ideas and unexpected connections in a "promiscuous mind".

"What the people have here is their passion and expertise. Writers explore every aspect of our lives, and books are traditionally how we both store and share everything we know," he told Reuters in an email.

"The festival just shoots off in new directions just following trains of thought. You never know where writers will lead you. They imagine the world in ways that challenge every previous assumption."

Former U.S. President Bill Clinton, who spoke at Hay in 2001, dubbed it "the Woodstock of the mind."

From humble beginnings - about 400 people attended the first festival - Hay has become a global brand, spawning festivals across the globe in cities from Cartagena in Colombia to Beirut and Dhaka.

Hay is built on books. There are more than 20 bookshops in the town, many selling second-hand volumes piled up in dusty basements. It is officially a Book Town - one of 14 across Europe and Asia that seek to make books a basis for sustainable rural development.

But it is not just the joy of rummaging through the shops or the eclectic nature of the programme that makes the Hay Festival unpredictable. Add to that the Welsh weather.

On the best days, readers loll on the grass with a book or sit in deckchairs licking ice cream or munching organic burgers. On others, local tractors are busy pulling cars out of the mud.

The year former U.S. Vice-President Al Gore came to Hay to talk about climate change, 2006, was pretty soggy but then so was the mythologised 1969 music festival Woodstock.

And while the throngs at Woodstock may (depending on the teller's recollection) have closed the New York State throughway, Hay keeps the crowds moving between the festival site and town centre with a series of shuttle buses.

EARTHWORMS, BOOKWORMS

The festival makes much of its efforts to be sensitive to the environment. There is an organic garden on site where earthworms meet bookworms.

Sales of books from UK publishers fell two percent in 2011 to 3.2 billion pounds but this total included a 54 percent increase in digital sales to 243 million pounds, according to the Publishers Association.

Military historian Antony Beevor, who has enjoyed international success with books including "Stalingrad" and "D-Day" and is a Hay regular, told Reuters the switch to e-books would affect literature festivals, even though their audiences were more likely to be loyal to the printed word.

"But whatever the changes to the way books are sold, festivals will still offer the best environment for debating ideas and historical arguments," Beevor said by email.

Hay 25 runs from May 31 to June 10. The first weekend, which coincides with a bank holiday, brings out the celebrities - comedians, prominent media folk and opposition politicians - in sunglasses or big rubber boots depending on the weather.

Apart from the authors, this year's line-up includes U.S. singer and civil rights activist Harry Belafonte, British jazz musician Courtney Pine and singer Ghostpoet.

To mark the London Olympics, there is a series of events devoted to Classical Greek culture. To mark the festival's 25th birthday a panel of 25 writers, economists and thinkers will be mulling 25 questions under the rubric "the way we live now" before a live audience and with online contributions.

Among the questions before panelists including Canadian novelist Margaret Attwood, "Wild Swans" author Jung Chang and environmentalist George Monbiot - "Which freedoms are you prepared to trade for greater security?" and "What makes you laugh?"

Hard economic times do not seem to have deterred visitors. Florence says numbers are up 18 percent on 2011 and he expects to sell 250,000 tickets to events this year.

"No matter how hard the austerity bites, people will always want to celebrate being together," he said.

(Editing by Paul Casciato)

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