Fines for overdue books, audio cassettes, periodicals and pamphlets were increased recently at Fairfax County public libraries and a new system was instituted to make current best-selling books more readily available.

The fines were raised from 2 cents to 10 cents a day for each item. The increase affects items borrowed Aug. 22 and thereafter. All overdue material returned before Sept. 12 will be charged at the lower rate.

The increase in fines, designed to encourage the prompt return of materials, is the first since the library was established in 1939.

Under the new system for best-selling books, all books listed in The Washington Post and The New York Times as current best sellers will be loaned on a first-come, walk-in basis only. The new procedure is to eliminate the delays in circulation of new books that previously were kept on hold for borrowers who reserved them in advance.

Library patrons will be able to continue making reserve requests for any other materials listed in the library computer's database. A 25-cent fee is being instituted for the service, which includes locating and holding an item, and advising the borrower of its availability. Requests to reserve items will be accepted only in person.

The library's direct loan system, under which materials available at one branch may be sent to another, will continue at no charge. However, current best sellers no longer will be included in the service.