About a mile east of the Capital Beltway on Leesburg Pike lies a sunken pyramid-style building laced with brick and glass block partly hidden by trees. But the 30,000 square-foot structure is about to become a visual affirmation that the public library network in Fairfax County is one of the region's preeminent systems.

The Tysons-Pimmit Regional Library, though nearly five months behind schedule, will open in September and join the new Reston Regional Library, also behind schedule, in bringing the county two steps away from completing its goal of providing a regional library in each section of the county.

Currently the system includes four regional libraries, which house 70,000 volumes or more, and act as central distributors to the county's 16 smaller community and mini-libraries. Additional regional libraries in Pohik and Centreville are due to open next year and in 1990, respectively.

The Reston Regional Library was designed by LBC & W of Falls Church and will replace the area's Hunters Woods Plaza location, with added space, books, seating, and study areas. The library will have an inventory of about 75,000 volumes, with a capacity of 150,000.

"Libraries are entering a new era," declared county library director, Edwin S. Clay III, who added, "they're more than places to check out books."

Tysons-Pimmit was designed by architects Mills, Clagett & Wening, of the District, and will also open with 75,000 volumes, and eventually expand to about 150,000 volumes. But the buildings' designs and content reflect the bold architectural dimensions and technological advances once reserved for the private sector.

Both buildings are one-story high, which will require fewer staff members to maintain than multilevel facilities, Clay said. The pyramid's ceiling in the reading room at Tysons-Pimmit spans nine to 30 feet high; lights line the perimeter, and pipes are fully exposed. The contour of the reading rooms at both libraries to a large extent will be defined by bookshelves, desks and chairs.

In addition to new services, the two libraries will expand on the tools that are available at the other regional libraries by adding more video cassette recorders, records, and cable and educational television programming that features everything from Sesame Street to MTV.

Tysons-Pimmit will become the new county distribution center of 16-millimeter films, which are currently housed in the administration's Springfield office. The building also has two conference rooms, one of which has a private kitchenette and a studio replete with sound-proofing and a control room that will among other things be used to produce and transmit cable television programs for Fairfax County Public Library Channel 44.

The price tag for these advanced centers of learning is $3.5 million each, money approved by taxpayers in a 1980 county bond referendum after a library board study in 1979 concluded that the county "Because the county is so large, the traditional concept . . . just wouldn't work." Library Director Edwin S. Clay would greatly benefit with seven regional facilities.

"Because the county is so large, the traditional concept of one main library and a bunch of pilot libraries like in the District just wouldn't work," Clay said.

Last year a survey conducted by the Public Library Quarterly found that Fairfax County ranked fourth with a circulation of 6.2 million books, out of 37 southern United States library systems with annual budgets of approximately $500,000.

The library network boasts 1.5 million books, 25,000 records, 4,200 video cassettes, and 2,398 films.

And in a commitment to guard against theft, the county is installing an electronic materials security system in all of its regional and community branch libraries.

Scheduled to be activated in July at the operational facilities, and later at the new libraries, the system is designed to ensure that books and other library materials are electronically checked out before they are removed from the library.

Officials hope that the $200,000 system will save the library from an estimated annual loss of 75,000 books with a replacement cost of about $900,000, Clay said.

"In addition to saving taxpayer dollars, the new system will help eliminate the frustration of not being able to find the particular books you need because they are missing," Clay said. "It should also prevent the loss of irreplaceable material."

Under the new system, patrons leaving the library will pass through an exit designed to sound an alarm if it detects any library material that has not been checked out.