Last year, you would have found Foster applying the same techniques to a pop-up book showing British actor Hugh Grant entertaining a Hollywood lady of the evening in the front seat of his BMW.

Pop-up books have grown up - fast.

After decades of focusing primarily on children's themes and being viewed by most as a children's format, pop-up books have entered an era in which anything goes, even - aaugh! - sex. Or as a young couple gasped as they opened The Pop-up Book of Sex in the humor aisle of a bookstore, only to have two naked bodies rise up off the page and into their faces: ''Oh, my God!''

Foster, one of about a dozen full-time paper engineers in the U.S., passed on the chance to work on that book, but he did tackle his first pop-up book for a mature audience last year: The Pop-Up Book of Celebrity Meltdowns was released this year by Melcher Media.

The book re-creates - in that in-your-face way only pop-ups can - such embarrassing moments as Janet Jackson's ''wardrobe malfunction'' at the Super Bowl, Paris Hilton's sex video and Michael Jackson dangling his baby boy off the balcony of a Berlin hotel.

''A pop-up book can be a children's book, but it is certainly not required to be,'' said Foster, who lives in Houston and was the paper engineer for Little Red Riding Hood, published in 2001, and Peanuts: A Pop-Up Celebration, published in 2004.

In the case of Celebrity Meltdowns, he said, ''the work we did has brought more laughter to people than any children's book I've been involved in.''

And it has had no negative repercussions on his career. Foster has been signed for several children's book projects since, including one being released next year in conjunction with a Disney movie.

Growing up on a farm in southern Louisiana, Foster missed out on pop-up books during his childhood. He went to the University of Tennessee, attended art school and worked for decades as a graphic designer before seeing his first one in the 1990s - Kees Moerbeek's Hot Pursuit, the story of a continual chase that can be read forward and backward.

Foster, fascinated by the three-dimensional, was instantly intrigued: ''From the first I saw time I saw it, I really wanted to do it. I thought it would be a great way to combine my three-dimensional urges with the skills I already had.''

With no formal training as a ''paper engineer'' - as those who design pop-ups are called - he taught himself by taking pop-up books apart.

''I destroyed a lot of them trying to figure out how they were done,'' he said.

Foster spent seven years working on pop-up books in his spare time. About a year ago, because of increasing demand for his services, he was able to quit his job as creative director of a packaging firm and work on pop-up books full time.

Ann Montanaro, director of information technology for Rutgers University libraries, is founder and director of the Movable Book Society, which was formed in 1992. It puts out a quarterly newsletter and holds an international conference every other year.

This year saw a continuation of the trend of pop-ups delving into adult themes, she said.

Montanaro has no problem with that - not even with The Pop-Up Book of Sex, which most bookstores keep behind the counter.

''The characters are nicely articulated, and they move nicely,'' she said. ''Actually, it's pretty awesome.''

Today, you can find pop-up books on everything from the alphabet to Alfred Hitchcock, from Bible stories to bondage, from Kwanzaa to Kama Sutra, from Smurfs to Stonehenge.

One of this year's more popular releases was Graceland, An Interactive Pop-Up Tour. Readers can tour the various rooms of Elvis Presley's Memphis, Tenn., mansion, pausing to open his refrigerator, thumb through his albums or change the channels on his TV.