Neuroscientist Mark Changizi explains how e-books, like much of the web, lack spatial navigability, which can be key to remembering information:
We don't navigate the web so much as beam hither and thither within it. Can't find your way to the ticket site? No matter, you can Google-beam directly there by typing in the name. And not only is the web not spatial or navigable, but the new reading experiences within documents have lost their spatial sense as well. … Need to jump to that part of the book where they discussed cliff jumping? You will get no help from the local topography, but you can beam yourself directly there via a within-document text search.
Screen size also matters:
[Jakob Nielsen, a web "usability" expert,] says that studies show that smaller screens also make material less memorable. "The bigger the screen, the more people can remember and the smaller, the less they can remember," he says. "The most dramatic example is reading from mobile phones. [You] lose almost all context."