Under sweeping, barn-like rafters, the reading rooms of Howard County's central library look like most others. Kindly, silver-haired women tend the reference desks, teen-age volunteers push carts through the stacks, and patrons rest in easy chairs with the latest bestsellers.

But this is no ordinary library. Here, overlooking the grassy flood plain of the Little Patuxent River, is one of the three or four busiest libraries in the country, a $4-million building that houses the latest in library technology, including a computer nicknamed "Gandalf."

The library, which opened in 1981, is a far cry from the second floor of a meat market near Ellicott City where Howard stored its public book collection in the 1950s. Today, there are weekend traffic jams outside the library as book-hungry motorists vie for a spot in its 120-space parking lot.

"Location is everything," joked Marvin Thomas, a 20-year veteran of Howard libraries and the central branch's director. "Then again, I'd like to think our popularity is also due to the service we've been able to provide."

Located on a knoll between Columbia's main street and busy Rte. 29, the library also benefits from its location in the middle of a highly educated, upwardly mobile community, Thomas said.

Columbians aren't the library's only patrons, but they and the residents of sprawling Ellicott City have transformed the county library into a flagship of the state system. "The whole point of a library is to give people the books they want," said J. Maurice Travillian, a library expert in the state education department. "There's no way to quarrel with the fact that the Columbia library has done that."

The Columbia library accounts for roughly 70 percent of the county's annual circulation of 1.5 million volumes; there are four smaller branches, one of which will triple in size by 1985.

"The circulation in Columbia itself is remarkable," said Robert S. Alvarez, who publishes a California-based magazine for library administrators. Only one other library--in Baton Rouge, La.--had a higher single-building circulation last year, Alvarez said.

And in Maryland, only Baltimore County has a higher per-capita readership, according to state officials. Last year, said Travillian, each of Howard's 130,000 residents read an average of 11.6 library books, compared with Baltimore County's 13.1. In Montgomery County, which Howard surpassed two years ago, residents read an average of 11 books from the libraries each year. Three out of every five Howard residents own a library card.

"Howard is a lot like a city library system, which is good in one way: It gets most of the books in one central place," Travillian said. "On the other hand, it can keep books out of a county's more remote sections. And Howard certainly has its more rural areas."

Under Thomas, who oversees 88 full-time employes and an annual budget of $2.1 million, the county maintains a network of three bookmobiles and a branch in Lisbon in the western part of the county.

"The service demands are increasing all the time," said Thomas, a soft-spoken New Yorker. "And we have the potential demand to go beyond the state of the art in libraries."

The central library already has a computerized card catalogue, and Thomas said he hopes to establish a video cassette library soon. He also hopes to outfit Ellicott City's Miller branch--which will grow from 7,000 to 22,000 square feet at a cost of $3 million--with desk computers patrons can use for research and book-hunting.

"We try to have as broad a scope as possible," Thomas said. "One consequence of our good service is that there may be fewer copies of a book around."

But, he added, "That doesn't appear to have affected our circulation."