As more U.S. schools adopt textbook-free teaching, some are questioning the quality of digital content and whether all schools have the resources to do it. Digital textbooks come in many forms -- from paid downloadable texts to free "open-source" content available online. "Keep in mind that with open-source materials, you have to ask, 'Where are they coming from?'" said Jay Diskey, executive director of the Association of American Publishers' school division. "Is it a trusted source? Is it aligned to state standards? Is it based on real research?"