"Colonel Mustard did it with the rope in the library." When many of us played the murder mystery game Clue as children, it seemed perfectly reasonable for an upscale home to have a book-filled library where family members spent time reading.

But today, libraries are increasingly out of fashion, perhaps going the way of that other now-obscure room on the Clue game board, the conservatory.

"When people walk into a modern home, they don't ask about a library," said Gopal Ahluwalia, director of research at the National Association of Home Builders.

In fact, separate space in a house specifically designated for reading "is fast becoming a rarity," said Mark Bauerlein, director of the Office of Research and Analysis of the National Endowment for the Arts, the government agency that last year documented a decline in book reading among Americans in a report titled "Reading At Risk: A Survey of Literary Reading in America."

Builders also have noticed the trend away from libraries. Adrian Edwards, a Reston-based custom home builder and chairman of the Northern Virginia Building Industry Association's Custom Builder's Council, builds eight to 12 houses annually that typically cost $2 million to $4 million each. He estimated that in the past 20 years, he has built only one "true home library" that didn't have a desk or computer in it.

Tim Carter, an Ohio-based syndicated columnist who runs the home improvement Web site www.Askthebuilder.com, said that in his reader mail over the past 12 years -- he receives more than 100 e-mail questions per day -- he has received no specific inquiries about a room used purely as a library, though many readers have asked about bookshelves.

Why are home libraries disappearing? For one thing, people are increasingly using space in homes for multiple purposes. "There is less dedicated space for any one thing," said Karen Harris, founding principal of Denver-based Architecture Matters and past chair of the Small Project Forum for the American Institute of Architects. According to that association's quarterly Residential Design Survey, conducted this spring among 600 architectural firms that concentrate on residential work, informal space is becoming more popular in homes, as is an open space layout with "fewer hard boundaries between rooms," said Kermit Baker, chief economist for the group.

When homeowners designate a spare room for one activity, it is usually for purposes other than reading. Since the 1980s, an increasingly common choice has been an exercise room, said the builder group's Ahluwalia. Custom builder Edwards has seen this firsthand. He said that 70 to 80 percent of the homes he builds contain a specific room for exercise equipment.

Home offices have also grown in popularity in the past two decades, said Ahluwalia. A 2003 study by his association found that 12 percent of new homes had a separate, designated home office. Typically those rooms contain a desk and some kind of bookshelves. Built-ins are an option that builders usually offer in more expensive houses, Ahluwalia said, noting that he keeps all his books in his home office. "There is a thin line between a home office and a library," he said.

Specified space to house high-tech entertainment, such as large-screen televisions, DVD players and computers, often referred to as a media room, has become highly desirable in the past decade. "People want that more than anything else," said Ahluwalia, explaining that "with people using 54-, 64- or even 84-inch TVs, you can't put those in an 8-by-10-foot room. You need at least a 12-by-15-foot room."

The shift away from home libraries is disconcerting to some, such as Bauerlein of the National Endowment for the Arts. Reading is "a high-concentration activity" that cannot easily take place while multi-tasking or in the presence of rooms with distractions such as e-mail, TV or computer games, he said. In addition, a home library typically houses many books, he said, and "the presence of books in a household has a deep effect on the habits, inclinations and intellectual development of people in that household."

In fact, in 1998, the Nation's Report Card, a nickname for a Department of Education reading assessment test of fourth, eighth and 12th graders, found that students who scored at the higher levels were more likely than their lower-scoring counterparts to have encyclopedias, magazines, newspapers and at least 25 books in their homes.

But builders say new homes have their own well-designed spaces where books can be housed and reading can take place. For example, a sitting room off the master bedroom can be used as a quiet reading area, suggested Linda Ellington, vice president of sales and marketing for Rockville-based Mitchell & Best Homebuilders.

Custom builder Edwards agreed. In the homes he constructs, the sitting room usually has built-in bookshelves and a gas fireplace. Reading in the sitting room makes sense for many people. "We are all so busy. When we finally wind down for the day" and have time to read, "we want to be near where we go to sleep," he said.

Another potential reading spot with few distractions is a reading loft, said Ellington. Available in some Mitchell & Best new houses, the loft is on the upstairs level of a home and takes away the optional two-story foyer. It is an open area large enough to hold books and a couple of chairs, Ellington said.

For some families, the absence of a library or other specific reading nook allows for the freedom to store and read books in a variety of spaces around the house. That's the case for Lisa Stacholy, principal of LKS Architects in Atlanta and vice chair of the American Institute of Architects Small Project Forum. She said her three children, ages 9, 6 and 3, love to read and own many books. They read in their bedrooms, in the lower-level art room, on the back patio and front porch, and even in the bathroom.

"We have books in every room," she said, noting that they are kept in built-ins, freestanding shelves and large plastic boxes. "It's not like I say, 'This is where the books are, and you have to sit here and read and put the book back before you take another one,' " she said.

A generation from now, Professor Plum may be committing his crime with a rope in the exercise room, the home office or the media room. But perhaps there will be a few books in each of those places.