As the Harvard Coop continues its crackdown on students who jot down book information in the store with plans to take their business elsewhere, undergraduates are devising new ways to skirt the Coop’s latest measures.

The 125-year-old bookstore has begun kicking out students who write down book identification numbers, known as ISBNs, because students can use those numbers to buy their books more cheaply online.

Last Thursday, Coop staff members, who have been asking anyone copying down book information to leave, called the Cambridge Police Department on three undergraduates working for CrimsonReading.org, a student-run Web site that allows students to compare textbook prices among online retailers, when they refused to leave for writing down ISBNs. Coop officials declined to comment over the weekend.

Rather than making group trips to the Coop, Crimson Reading, which is in its third semester of operations, has now opted to use “crowd-sourcing,” relying on student and faculty submissions of ISBN numbers.

About a dozen Crimson Reading staff members and volunteers had been relying on stealthily writing down the ISBN numbers on small notepads and index cards, said Crimson Reading’s director, Jon T. Staff V ’10.

“They got pretty wise to us and others looking to compare prices on books,” he said, adding that he has been asked twice to leave the Coop textbook floor. “They’ve had a pretty sharp eye on it.”

So far, about 50 students and 50 faculty members have submitted ISBN numbers to the Web site, according to Crimson Reading co-founder Tom D. Hadfield ’08.

Yesterday, Undergraduate Council President Ryan A. Petersen ’08 wrote an e-mail to the student body encouraging students to submit ISBN numbers for courses.

Other students, Staff said, have been relying on more high-tech methods to bypass the Coop staff’s methods, such as text-messaging ISBN numbers to themselves, using phones to take photos of the books, or using computers to copy information.

“I was at the Coop and pulled out my laptop to look online at books I needed to buy,” Joshua G. Allen ’09 said. “I was planning on buying them at the Coop and returning them later.”

According to Crimson Reading’s calculations, the Coop’s textbook prices last semester were 23 percent higher on average than the lowest online retailer’s prices, said Hadfield, who is also a member of The Crimson’s editorial board.

Nevertheless, the Coop will always have at least one advantage over its online competitors—convenience.

Browsing the Coop’s top floor yesterday, Siddhant S. Singh ’11 sighed while looking at the price tag on a multi-variable calculus textbook.

“I got my Ec 10 textbook for $24 on Ebay,” he said, referring to the introductory economics text by Beren Professor of Economics N. Gregory Mankiw, which sells for $167.95 on Amazon.com. “But it takes time, and I need this book now.”

—Staff writer Anna L. Tong can be reached at tong@fas.harvard.edu.