The D.C. Public Library, concerned with the loss of tens of thousands of unreturned books, has hired a collection agency.

But don't expect to receive letters or telephone calls threatening court action for overdue fines and books.

The collection letters are primarily to remind borrowers to return the books, said Kurt Shutter of the local office of G.C. Services of Houston, the agency hired by the library system 14 months ago.

The letter informs the borrower that "borrowing privileges are suspended" and that return of the material "will lessen the cost of new fines," Shutter said, but it does not mention collection of fines or book payments and is "mild" compared to many commercial collection letters because the library "is sensitive to public opinion and afraid to come on too aggressively."

"You always have people who take advantage of the system, but they are a minority," said Hardy R. Franklin, director of the main Martin Luther King Jr. Memorial Library, the 20 library branches, four community libraries and three kiosks. "We're not in the business of punishing people. We sort of go on faith that the borrower will return the material."

Library officials said their aim is to get books back, not to collect fines, because there is a maximum charge of $3 for an overdue book borrowed by an adult, regardless of how long the borrower has had the book. The maximum fine for children's books is $1.

But despite the small fines, some books have not been returned. Last summer library officials dropped from their computer system the titles of 39,000 books that had been missing for three years and presumed unreturned.

The purging represents 3 percent of the city's collection of 1.4 million books, officials said.

The library has paid G.C. Services, one of the nation's largest collection companies, $3,000 to send some 5,300 letters to patrons with books more than six weeks past due and worth as least $15 each. Library officials said it would be too expensive for the city to try to keep track of overdue accounts.

Officials believe the investment, at more than 50 cents a letter, has paid off, but that is an assumption based on an experiment the library ran with the agency in 1983.

According to that experiment the agency's letter resulted in closing 4 percent more accounts than without the letter, said Houston Maples, director of automated services for the library.

The collection agency's letter improved by more than 7 percent the number of accounts that were partially cleared, either with books returned or with fines paid, he said.

For the library, that translated into a gain of nearly $4 worth of books and fines for each dollar spent on the agency letters, Maples said.

According to District budget figures, last year the library collected $100,000 in fines, payments for lost books and replacements fees for lost library cards -- double the $50,000 collected in each of the previous two years.

"The 50-cent letter is as quick and efficient an action as we know," Maples said. "Following up on these people is very expensive, as these things go."

The library first hired a local collection agency in 1981 but found that it was too expensive.

Many of the unreturned books are borrowed by college students who check out District library books assigned as texts for classes and keep them all semester to avoid buying them, Franklin said.

While the library is aware that students take advantage of its low fines, Franklin said he and the library's board of directors agree that stiffer fines would encourage borrowers to keep the books instead of returning them.

"We prefer our method of raising the consciousness of the public," Franklin said.

"It's their responsibility to return the material."