Slowly, but in increasing numbers, grade schools across the country are supplementing or substituting the heavy, expensive, and indelible hardbound book with its lighter, cheaper, and changeable cousin: the digital textbook, reports the Washington Post. Some versions must be purchased; others are "open source"–free and available online to anyone. Some praise the technology as a way to save schools money, replace outdated books and better engage tech-savvy students. Others say most schools don’t have the resources to join the digital drift, or they question the quality of open-source content. Hardbound books still dominate the $7 billion U.S. textbook market, with digital textbooks making up less than 5 percent, according to analyst Kathy Mickey of Simba Information, a market research group. But that is changing, as K-12 schools follow the lead of U.S. universities and schools in other countries, including South Korea and Turkey. In Florida’s Broward County, students and teachers log online to access digital versions of their Spanish, math, and reading books. And in Virginia this year, state officials and educators unveiled a free physics "flexbook" to complement textbooks…