Possibly controversial suggestion - but why have any physical books at all?
...
Because ...
a) They don't need batteries.
b) I can scribble notes / code fragments on them as I read through which improves / reinforces my understanding of the material being presented.
c) Physical bookmarks, i.e. scraps of paper, are easier to deal with than electronic ones.
d) I find it easier to read dead tree editions because I can flip back and forth.
Having said that, tech. books do go out of date frighteningly fast and if you're comfortable with PDF et al it's no skin off my nose.
Coincidentally, I have, in the last week, put nearly all my C, C++ and early version C# manuals in the paper recycling box. The Java & Android books may be joining them, although I am currently undecided about that. One of the C books was over 30 years old!