Firestone Library, believed to be the last private research library in the nation to give the public free, unrestricted access to its stacks, began charging for that privilege today.

The fees are an inevitable consequence of spiraling costs, a tight budget, wear and tear on books and facilities and an increasing amount of theft, Princeton University librarian Donald W. Koepp said.

Similar problems are increasingly afflicting other libraries. According to the Chicago-based American Library Association, the new fees and other user restrictions at Princeton were already in place at virtually all of the nation's other private, and an increasing number of public, research libraries.

Julie Carroll Virgo, executive director of the ALA's Association of College and Research Libraries, warned that medium-sized college and public libraries may be next "if they don't have the resources to protect their collections or to keep their stacks in shape."

Koepp said, "We're seeing the end of an era. We all regret it, but we don't see that there's any practical alternative."

"Access cards" for the Princeton libraries were on sale last week at $20 for a month or $75 for a year; and turnstiles had been installed.

Critics of Firestone's new policy questioned the need for it.