NEW YORK — The number of books threatened with removal from library shelves dropped last year to its lowest total on record, with 405 challenges reported to the American Library Association.

The ALA has been tracking efforts to pull texts since the early 1980s, when it helped found Banned Books Week as a celebration of free expression. The 25th annual "Banned Books" program is taking place this week, as libraries and bookstores highlight works that have been removed or faced removal.

Challenges have gone up and down over the past few years, but overall have dropped by more than half since Banned Books Week was started. Judith Krug, director of the ALA's Office for Intellectual Freedom, cited a couple of possible factors for the decline: Librarians are better prepared to organize community support on behalf of a book, and would-be censors are focusing more on online content.

"There's only so much energy to spend on situations or concerns outside the home," Krug told the Associated Press during a recent interview. "A large majority of our challenges deal with what children are reading in schools and many adults are now so concerned about what's on the Internet that they have refocused."

The ALA defines challenges as "formal, written complaints filed with a library or school requesting that materials be removed because of content or appropriateness." For every challenge listed, about four to five go unreported, according to the library association, which sponsors Banned Books Week along with the American Booksellers Association and others in the publishing community.

The number of works actually pulled has also decreased over the past quarter century, from more than 200 in 1982, to at least 44 last year, including Toni Morrison's The Bluest Eye, the Nobel laureate's debut novel. The school board in Littleton, Colo., ordered the book removed, largely because the story includes the rape of an 11-year-old girl by her father.

"The purpose of Banned Books Week is to bring to everyone's attention that our First Amendment rights are fragile and that somebody out there will take them away from you if you're not careful," Krug said.

The most "challenged" book of 2005 was Robie Harris' It's Perfectly Normal: Changing Bodies, Growing Up, Sex, and Sexual Health, a guide for middle school students. Others high on the list include such perennials as J.D. Salinger's The Catcher in the Rye, Judy Blume's Forever and Robert Cormier's The Chocolate War, all cited for "sexual content" and inappropriate language.