By Catherine Hornby

FRANKFURT, Oct 7 (Reuters) Bestselling books don't always make great blockbuster films, agents and producers warned authors selling rights at the Frankfurt Book Fair this week.

In fact, most titles acquired by the production companies prowling the fair are more likely to gather dust on a studio shelf than appear on the silver screen, they said.

''For every hundred books optioned for movies, I doubt that more than five get made into films,'' said Julian Friedmann of Blake Friedmann Literary, TV and Film Agency.

Successful film adaptations of novels, like Dan Brown's ''The Da Vinci Code'', can pull in huge box office receipts and make enormous profits. ''The Da Vinci Code'' earned more than 0 million alone from ticket sales worldwide.

That is why filmmakers often pounce on popular stories that have proven market appeal. But the difficulties of adaptation often mean that films of bestselling novels have more to live up to and are the hardest movies to make.

Two of the latest bestsellers to be adapted to film, ''The Devil Wears Prada'' and ''Perfume'' have had some success at the box office.

But Tom Tykwer's adaptation of Patrick Sueskind's ''Perfume'' was shunned by critics despite the public accolades which pushed it to the top of the German and Russian box office charts. David Frankel's film version of Lauren Weisberger's 2003 book ''The Devil Wears Prada'' earned 6 million, but had mixed reviews.

Thrillers and comic books are often the easiest to translate into film, movie executives said. Literary novels often require far more editing, including the complex task of fleshing out characters who are already alive in the minds of readers.

''A screenplay has around 20,000 words in relation to some novels, which have more than 100,000,'' screenwriter Bernd Lange said.

The scarcity of optioned books actually making it to film has frustrated authors for years.

This year, the world's oldest and biggest international gathering of publishing executives is working more closely with cinema and handing over a day to the Berlin International Film Festival for screenings and workshops.

''Film rights often go into a black hole,'' producer Danny Krausz said. ''Production companies take the rights but they often don't have the talent to see it through.'' REUTERS PDM HT0940