Banned Books Week, which will be observed Sept. 20-27 this year, has a formidable task in educating the public, says a UC Davis English professor who teaches about censorship.

That's because challenging books is a pervasive American response to literature that deals with sex, race, homosexuality and other uncomfortable subjects, says Don Abbott, who specializes in freedom-of-expression issues.

"There is this impulse in American society to challenge literature because some groups want to prohibit people from reading certain books, which is a violation of the First Amendment," Abbott says.

In particular, community groups are interested in shielding junior-high and high-school aged children from various subjects.

"It's not always a parent group that is concerned but a local group without direct ties to students," Abbott says. "They are often worried about books with too much sex and with religious ideas deemed inappropriate."

Among the most challenged books these days are those in the Harry Potter series, because they deal with wizardry and magic, Abbott points out. However, a perennial favorite book to ban has been "Huckleberry Finn," because of its presumably racist message and language.

The American Library Association, which sponsors the Banned Books Week and compiles an annual list of challenged books, draws distinction between books that are banned -- taken off the shelf -- and those that are challenged with some action taken to make the book more difficult to obtain.

Most of the challenged books have received recognition as good or even great literature, Abbott says.