Enterprising thieves inconvenience patrons and cost taxpayers thousands each year stealing books, CDs, DVDs and other materials from public libraries, usually intending to sell them later.

Maggie Killackey, a Chicago Public Library spokeswoman, said anti-theft measures are in place, including embedded security tags used at all 79 library branches. Sensors are magnetically charged until checkout, when they are deactivated. Gates at the door detect tags that are still charged and sound an alarm.

"Everyone that leaves the branch has to go through those detectors," Killackey said. "Any attempts at theft (are) exceptionally low."

Susan Strunk, Naperville Public Library deputy director, said there are a number of reasons library materials are tagged as missing.

"Sometimes it's misshelved, and we mark it missing," Strunk said, adding that audiovisual materials are most noticeable when they are taken.

Strunk added that she was pleased with the low theft rate at Naperville's three libraries. A systemwide inventory last August showed that out of the 730,500 items in the library's collection, 0.6 percent, or 4,383 pieces, could not be found.

"There wasn't as much as we might have thought that would be missing," Strunk said.

Assuming that each missing item was worth $20, it would cost Naperville taxpayers more than $87,000 to replace the missing materials.

Strunk would not explain in detail what kind of security the library system has in place but did mention that Naperville libraries use cameras and patrolling staff monitors to watch for suspicious activities.

A 2004 survey of 17 libraries in the North Suburban Library system, a partnership of more than 700 public, academic and corporate libraries in Cook, Lake, Kane and McHenry counties, found that only six had security cameras installed.

Once thieves leave the library, they can either keep the item for themselves or attempt to resell it to used book stores or on the Internet. The American Library Association recommends libraries stamp their property with a unique identifier that ties the materials to an individual library.

Stolen books turning up for sale on Internet auction site eBay are not unheard of either. The policies at eBay do not explicitly prohibit ex-library books from being listed for auction, and buyers have no way of telling if they are receiving a stolen book until it arrives in the mail. The company urges eBay users to call the police to report items they suspect might be stolen.

Despite watchful librarians and more advanced security, some unscrupulous patrons still manage to steal library property.

"It happens, but not terribly often," said Strunk. "Once in a while we seem to have somebody who's determined to make off with them."