I remember reading a study that concluded retention from reading on a Kindle / e-ink device was lower.

They concluded that the physical differences of books (size, weight, feel, cover) may help to form memories.

I can't say the above is proven fact. Citation definitely needed, but I find it a compelling idea, and would like to learn more.

Incidentally efficient language learning also uses somewhat superfluous activities to help create memories and improve retention. Our brains do seem to thrive off stimulation from multiple sources.