By 1985 Prince William County residents may be able to check out library books at their local shopping center if a long-range library plan endorsed by the Board of Supervisors is implemented.

The county is applying for a $300,000 federal public-library construction grant to build six mini-libraries on shopping center parking lots and other sites throughout the county.

The small libraries, which will hold about 3,000 books each, would serve as a supplement to the two full-service libraries already in existence in the county, officials say.

County library director Mary Jo Detweiler said the two existing public libraries--a main library in Manassas and the smaller Potomac library in Woodbridge--are inadequate for the combined 170,000 residents of Prince William, Manassas and Manassas Park.

County supervisors said they may build the mini-libraries, at a cost of $95,000 each, even if the county is not awarded the federal grant.

Next month, they are expected to vote on a proposal to include the mini-libraries in the county's long-range capital plan, resolving to appropriate up to $1 million in county funds for the cost of construction and books.

"We've been looking at our library space every year for years," Supervisor Chairman Kathleen K. Seefeldt said last week. "What the citizens want are the smaller satellite libraries where they can come in and browse."

Detweiler said mini-libraries are considered the wave of the future in the public book world.

Fairfax County has a 500-square-foot mini-library in the Fair Oaks Mall and two small libraries in Great Falls and Burke Center that seat only 32 people each.

Fairfax officials also are considering building mini-libraries near the local stops on the Metrorail line, county library director Edwin S. Clay said. He said the county is pleased with the popularity of the smaller libraries and wants to build more.

The Fair Oaks Mall library, opened in 1980, is used by thousands of residents and is less expensive to operate than the county's regional libraries, Clay said.

He said it costs the county 33 cents every time a book is checked out of the Fair Oaks kiosk compared with $1.32 for every book checked out of the county's regional libraries.

"More importantly, we are getting facilities to where the people are," he said. "That is the most important thing for libraries."

The mini-libraries planned for Prince William would be 170 square feet and hold 2,000 to 3,000 books each, Detweiler said. However, residents would be able to order and pick up regional library books at the mini-libraries.

Detweiler said Prince William has suffered from inadequate library space for years, as the county's population growth bounded past that of its services.

In 1981, county residents defeated a $5 million referendum to build a large regional library in the eastern end of the county and another smaller library in Gainesville, at the western end.

"Since then we've been evaluating alternatives," Seefeldt said. "Something definitely needed to be done."

A county library task force even briefly considered opening school libraries to the public but rejected the idea for security reasons, officials said.