WASHINGTON (AP) _ James H. Billington, welcomed by President Reagan as ''America's librarian,'' says he takes the helm of the Library of Congress as a lover of books but with an open mind about new forms of technology to replace them.

''The trick is to retain the old values as you accept the new things,'' said the 58-year-old scholar, sworn in Monday as the 13th librarian of Congress in the library's 187-year history. ''I see a great future for the book.''

Billington, a former history professor who has been director of the Woodrow Wilson International Center for Scholars in Washington for 14 years, succeeds Daniel J. Boorstin, also a historian, who was appointed librarian by President Ford in 1974.

''Jim has had a life-long love of books,'' Reagan said at a swearing-in ceremony in the ornate Great Hall of the library's 90-year-old Renaissance style Thomas Jefferson Building. ''He stands here today because of an appreciation for scholarship instilled in him by a father who never went to college, but who filled his home with books bought second-hand to save the family's limited funds.''

''We entrust you with this great natural resource, Dr. Billington, and we are proud to have you as America's librarian,'' the president said.

Meeting with reporters later in the librarian's office in the gleaming white James Madison Building across the street, which was opened seven years ago, the Philadelphia-born Billington said of his father, ''Even though he had no formal education, he had wisdom, and it came from books, and it was transmitted to people, and that's what this place has got to be about somehow.''

''It's a little harder to do it than it was around Leary's book store, but I hope we can make a start.''

Asked whether he approached decisions about information technology with a bias in favor of books, Billington said, ''I certainly hope so. I wouldn't trust anybody to introduce a technology that was going to replace books who didn't love what was being replaced.''

''At the same time,'' he said, ''I wouldn't want anyone making these decisions who didn't have an open mind and a certain amount of little boy's excitement about the new technologies.''

''There is a bias in favor of the book which does not mean a closed mind toward new technology but an insistence that the new technology explain what it uniquely can do, and it uniquely can do a great many things,'' he said.

''But as far as organizing and illuminating the human mind is concerned, the book has played an absolutely extraordinary role.''

In recent years the Library of Congress, the world's largest library with a rapidly growing collection of 84 million items, has been a leader in use of computers, laser discs and other technological developments for the storage and retrieval of information.