SAN FRANCISCO (AP) _ The nation's largest book distributor was accused by the federal government Monday of overcharging thousands of schools and libraries by up to $200 million.

The Justice Department joined a whistle-blower suit filed against the distributor, Baker & Taylor Inc. More than 90 percent of the nation's 15,000 public libraries buy some or all of their books from Baker & Taylor.

The lawsuit was originally brought by the former head of the Richmond, Va., library system and a former Baker & Taylor salesman. They accused the company of changing the coding on discounted medical and legal texts and overcharging $2 to $10 more per book.

Baker & Taylor, based in Charlotte, N.C., did not immediately return calls.

The lawsuit estimated that the overcharging cost schools, libraries and other public institutions $100 million to $200 million over 10 years, ending in 1993.

The federal government could recover money for books sold to the military and the Library of Congress, and possibly for books bought by institutions using federal grants.