Book publishers are stepping up efforts to stop college students from downloading illegal copies of textbooks online. One Web site, Textbook Torrents, promises more than 5,000 textbooks for download in PDF format, complete with the original books' layouts and full-color illustrations. Users must simply set up a free account and download a free software program that uses a popular peer-to-peer system called BitTorrent. Other book-download sites are even easier to use, offering digital books at the click of a mouse. In response to such sites, the Association of American Publishers hired a law firm this summer to scour the Web for illegally offered textbooks. The firm has identified thousands of instances of book piracy and has sent legal notices to Web sites that host the files, demanding their removal. The group is looking for all types of books, although trade books and textbooks, which generally have high price tags, are the ones most frequently offered on peer-to-peer sites. So far the publishing group has not sought legal action against individual student downloaders, as the Recording Industry Association of America has done in its campaign to stamp out illegal music trading at colleges.