This source has charts about the weight of textbooks over grades and subjects.

A Best Practice Review prepared for the
Joint Information Services Committee (JISC)

Exposure Version, November 24th 2008

Peter James and Lisa Hopkinson

Higher Education Environmental Performance Improvement Project, University of Bradford
SustainIT, UK Centre for Economic and Environmental Development

Then the university ran a pilot study with the Sony Reader, a device much like the Kindle. University officials learned some sobering lessons about electronic books. Students who got the machines quickly asked for their printed books back because it was so awkward to navigate inside the e-books. Mr. Hubbard still dreams of lighter bookbags and lower costs, but the university is now moving more slowly--and running tests involving several different types of e-books. Based on the author's talks with professors, students, and administrators at Northwest Missouri, lessons for any university considering assigning digital textbooks include: (1) Judge e-books by the device and software used to display them; (2) There will be a learning curve; (3) Professors are eager students; (4) Battery life is a major consideration; (5) Subjects are not equally e-friendly; and (6) Environmental impact matters.