LONDON (Reuters) - He caught the music world napping in January with his first new song in a decade and soon had critics searching for superlatives to describe his new album "The Next Day".

The next big question for David Bowie and his remarkable comeback is whether the element of surprise and subsequent acclaim will turn into record sales.

"The Next Day" is in stores on Monday in Britain, where industry watchers are confident it will top the album charts, and on Tuesday in the United States, where the "Space Oddity" singer has enjoyed more patchy success in the past.

It is already available in other key markets, and the early signs are that the 66-year-old master of reinvention has a hit on his hands.

According to his official website, the deluxe version of the recording went to No. 1 on the digital iTunes album charts in 11 of 12 countries where it was released on Friday, including Australia, Germany and Sweden.

"There has been a lot of interest in both the social and traditional media which will connect not only with the established fan base but also with younger fans," said Gennaro Castaldo, head of press at British music retailer HMV.

"As a campaign, I can't think of many that have been more brilliantly orchestrated," he added.

Ironically, part of that "campaign" has been for Bowie to remain invisible, allowing collaborators like producer Tony Visconti to tell the media about how the star's first studio album since 2003's "Reality" came about.

So rare had sightings of the "Starman" become in New York, where he lives, that articles appeared in the British press late last year speculating the "recluse" had unofficially retired.

"GRETA GARBO OF POP"

Simon Goddard, author of new Bowie book "Ziggyology" published by Random House imprint Ebury, said his mystique was a part of the appeal, and showed that his interest in music far outweighed any appetite for the trappings of celebrity.

"He released two albums in the very early 70s featuring covers of himself in poses inspired by Greta Garbo," Goddard told Reuters.

"Fast forward three or four decades and he becomes a rarely-sighted paparazzi quarry living in New York ... He engages with the media on his strict terms because he's surpassed any desire to engage otherwise. His art is all the engagement he needs."

Bowie, who has shunned the limelight since he suffered a heart attack on tour in 2004, last performed on stage in 2006. It was with a sense of shock that his fans woke up on January 8, his 66th birthday, to the news he had released a new song.

"Where Are We Now?", a melancholic look back to the time Bowie spent in Berlin in the 1970s, was the first single from "The Next Day", followed weeks later by "The Stars (Are Out Tonight)".

Both came with inventive videos which baffled as much as they entertained, affirming that Bowie was still the enigma who wowed the pop world in the late 1960s, 70s and 80s with glam-rock, androgynous alter egos and a radical sense of fashion.

Critics had barely a bad word to say about the 14-track album, with the Independent's Andy Gill calling it possibly "the greatest comeback in rock'n'roll history" in a five-star review.

Alexis Petridis, writing in the Guardian, said: "Listening to it makes you hope it's not a one-off, that his return continues apace.

Whether the return will include live performances remains to be seen, although Bowie's guitarist Gerry Leonard whetted appetites when he told Rolling Stone magazine he thought it was "50-50" Bowie would tour again.

Author Goddard attempted to sum up the level of excitement that has accompanied Bowie's return.

"Bowie's appeal has lasted because his influence is fundamental to everything that we in the 21st century understand as pop music," he said. "Remove Bowie and pop's whole house of cards as built up over the last 40 years or so collapses."

Bowie's impact on modern music matched that of The Beatles - and the only contemporary star to combine music and art to the extent he did in the 70s was Lady Gaga, said Goddard.

"The hysteria is justified," he added.