An archivist and an antique bookseller were charged Friday with stealing millions of dollars' worth of rare books, illustrations, maps and photographs from a Pennsylvania library over a 20-year period, including Isaac Newton's "Principia," considered a watershed of science.

In some cases, the archivist used an X-Acto knife to cut potentially valuable maps and illustrations out of books in the Carnegie Library of Pittsburgh's historical collection, authorities said. They ultimately sold to other collectors famous originals that were hundreds of years old, authorities said.

Researchers found mor...