Here's an daily mail article about it and here's the actual study.
Actual study wasn't at all about e-Readers, but actually about fonts. Fonts that are harder to read like Comic Sans (that one is for all you Comic Sans haters!) and others make people to remember better the actual content of the text. Surprisingly it makes people to less confident that they have learned the content, but actually still people learn better.
It's amazing how our brains and mind works. When reading is harder and you are less confident about your learning, you have learned better. That's a good tip for all book creators. If you want people to remember your thoughts better, you might want to use some bit harder to read font.
Written by +Henri Hämäläinen