Loudoun County officials broke ground Saturday on the county's first regional library, an $11.5 million project that will serve as Loudoun's primary reference center when it is completed late next year.

A special design feature will be a drive-through window, a first in Northern Virginia. Patrons may call ahead to order a book and then drive up to check it out.

The Eastern Loudoun Regional Library, to be built on 7.5 acres in the Cascades development along Route 7 in Sterling, is the centerpiece of the county's efforts to develop a modern library system. By early 1992, that system should contain a new Leesburg library and an addition to the Purcellville Library as well as the Eastern Loudoun Library. The first two parts of the system, libraries in Middleburg and Lovettsville, opened this year.

In areas still served only by old libraries -- Sterling, Leesburg and Purcellville -- patrons often wait for weeks to get a book. Thousands of books are in storage. Reading space is at a premium while meeting space is non-existent. The new library system, funded by a 1984 bond referendum and local tax dollars, is designed to alleviate that crowding.

"The need is so great," said Sally Hunt, director of the library system. "There's so many people being served by a very small library."

Library proponents fought a successful battle last spring to keep funding for construction of the new Leesburg library and the addition to the Purcellville Library in this year's county budget. County Administrator Philip A. Bolen had proposed delaying both projects to save money, which proved to be an unpopular idea across the county.

"It would have jammed this {new Eastern Loudoun} library with an overabundance of people needing help, if those other libraries were not built," said Linda Conti-White, chairman of the Eastern Loudoun Library Advisory Board.

The new library is meant to serve the whole county, library officials say, but they expect people to use their neighborhood libraries when possible. Patrons in other parts of the county will also be able to get information from the Eastern Loudoun Library by telephone, computer and facsimile machine when they visit neighborhood libraries.

The Eastern Loudoun Library, with 30,000 square feet of space and seating for 200, will be the largest in the system. It will contain 140,000 books, including the county's largest collection of children's and reference books, as well as videotapes, compact discs and microcomputers. An on-line computer data base, now housed at the Thomas Balch Library in Leesburg, will give county librarians access to state, regional and national information systems.

"The libraries in general are going to be taking a big step where they've never been," said Keith H. Raggio, a member of the Loudoun County Library Board of Trustees. The Eastern Loudoun Library will be "what we forsee the services of the library to be in the future of the county," he said.

The library, designed by Richmond architect Richard J. Fitts, will be a one-story red and cream brick building on a wooded site donated by Kettler & Scott, developer of Cascades.

"It's tailor-made for that population in eastern Loudoun," Hunt said. "A lot of commuters and busy people with young children."

The library will also contain a public meeting room complex with two 120-seat rooms and a children's area with a special room for story hours and craft workshops.

"It's going to be a gathering place," Hunt said.