Publishing giant Penguin Group on Tuesday stopped adding digital works to US library shelves where they can be borrowed by owners of Kindle electronic readers. In a statement, Penguin cited unspecified "concerns about security" as the reason for delaying release of new titles in digital formats to libraries. Digital book distributor OverDrive.com said in a blog post that it was instructed by the Britain-based publisher to suspend availability of Penguin e-books and disable "Get for Kindle" features in all Penguin titles. "We remain committed to working closely with our business partners and the library community to forge a distribution model that is secure and viable," Penguin said. Seattle-based online retail titan Amazon recently made it possible for Kindle owners to borrow books for the e-readers from more than 11,000 US public libraries. With Kindle Library Lending, Kindle books can be checked out from a local library for reading on any Kindle device or free Kindle applications for Apple, Android, Blackberry and Windows devices or personal computers and Macs. Early this month, Amazon opened its own lending library of digital books available free to owners of its popular Kindle tablets. To borrow from the Kindle online library people must be members of Prime, a premium Amazon service that provides free two-day shipping on all items purchased and unlimited streaming of online films and television shows. The annual subscription fee for Amazon Prime is $79. The library boasted thousands of works including many from New York Times best-seller lists. People can borrow one book each month. Amazon worked out deals to pay publishers fees for lending books in the company's library, raising the possibility that Penguin is interested in similar compensation for digital works borrowed from public libraries.