Although if you are after focus and depth, arguably you would be reading some sort of primary source rather than an encyclopedia. I've definitely been at a library, had a quick question on something tangentially related to what I'm reading, and pulled up Wikipedia on my phone to quickly look something up. I wouldn't want to read a whole book on a phone, since I agree with you that I can more easily absorb information from an actual book. That being said, looking up something on my phone is far less distracting than having to get up, put the book down, and go over to look something up in an encyclopedia or dictionary.