"The degree of unaffordability is getting to the point that it's hurting learning."

College student Caitlin Ryen works two jobs to support herself and pay tuition at South Florida Community College. When a new semester rolls around, that usually means choosing which textbooks to buy, or not buying any at all, she says.

The 22-year-old caught a break this term when she found out her physics course materials were free. Instead of asking students to buy a textbook from a major publishing company, Ryen's professor assigned a free digital textbook that he customized for the class.

Ryen's professor, Erik Christensen, began exploring the format in 2007, when a student asked to borrow a textbook because he couldn't afford one. While looking around for affordable alternatives, Christensen came across the concept of open-source textbooks, or course materials offered for free online by their authors under a nonrestrictive license.

Read more at CNN.