Madrid, Spain Spanish writer Andres Trapiello is used to painstaking research at his desk, and he needed all his scholarly patience for his latest work: adapting Spain's most revered novel, "Don Quixote".

Labouring to render in modern Spanish this 17th-century comic masterpiece about a delusional would-be knight, he stumbled on an unfamiliar word: "trompogelas".

"It was absolutely unintelligible," Trapiello told AFP. After a whole morning's research, he decided it meant something like: "It went in one ear and out the other."

Spaniards are crazy for "Don Quixote". They often quote the tale of the bumbling country gentleman and his tubby sidekick Sancho Panza, whose misguided adventures are known to readers worldwide.

But as they mark the fourth centenary of the novel's completion and the author's death, sceptics are asking: how many Spanish-speakers can actually read the original 1,000-page opus?

- Bad school memories - 

In a survey published in June by Spain's state research institute CIS, nearly six out of 10 people in Spain said they had read at least part of Don Quixote or some adaption of it.

But more than half said they thought the book was a "difficult" read.

"There are a great many people who have not read it or who have given it up several times because it is so hard," said Trapiello.

"They are obliged to read it in language that is not understood nowadays. They make you read it at school and lots of people have bad memories of that."

- 'Crime against literature' - 

In late July Trapiello's adaptation stood ninth on the Spanish bestseller list published by ABC newspaper -- two places behind "Grey", the latest erotic romance by British author E.L. James.

The Spanish Royal Academy, the official authority on the language, last year published its own simplified version for schools by the historical novelist and scholar Arturo Perez-Reverte.

Some intellectuals lashed out at what they saw as tampering with their beloved Quixote.

David Felipe Arranz, a literary journalist and academic at Madrid's Carlos III University, called the new versions "a crime against literature".

"I ask the booksellers in Madrid and they tell me no one buys Cervantes' original novel anymore because readers prefer the 'light' version," he told AFP.

"You cannot twist the flavour of the words of the greatest writer in our language."

- Quixote's world - 

Cervantes' comic tale marked world literature forever.

The Cervantes Institute, Spain's network of overseas cultural missions, say it is reckoned to be the most translated book after the Bible.

It has been rendered in 145 languages and dialects and adapted into comic books and children's stories, said Ernesto Perez, the head of cultural activities in the institute.

"What a paradox," said Trapiello. "A French, German or English-speaking reader can read it without any problems" in translation.

"But Spanish-speaking readers can't understand half of it unless they read it with notes."

 - Sexing up Cervantes - 

The first volume of "The Ingenious Gentleman Don Quixote of La Mancha" was published in 1605. 

Within a few years it had been translated into English, with characters' speech adapted into Cockney slang, and into French, with added sex scenes made up by the translators, Perez said.

The second part came 10 years later and Cervantes died in poverty a year after that in 1616.

Authorities said this year they had identified his remains in the crypt of a convent in Madrid.

At the Cervantes Institute's palatial headquarters in Madrid, display cases are filled with elegant foreign editions of Quixote.

The institute has laid out its collection of translations, along with posters and a screen showing film versions, in an exhibition that runs until September.

Meanwhile, branches of the institute around the world have been hosting Quixote-themed talks, readings and film screenings.

The Royal Academy this year published a new standard edition of the text with footnotes and essays.

The last such edition in 2005 sold three million copies, said Francisco Rico, head of the academy's series of classic titles. He welcomes other adaptations.

"Any attempt to make Quixote more readable is positive," Rico said.

Although Quixote "at first was not highly rated intellectually", it has been consistently popular with the public, Rico said.

"Shakespeare got forgotten for a time. For centuries no one remembered Dante. Voltaire has had his ups and downs. But Quixote has been a bestseller ever since it was published," he added.

"And what's more, in all languages."