Two things. First, go for the Kindle Voyage; it's just out, and it's a bit better than the Paperwhite.
As for a book: get the Shobogenzo:
I actually don't recommend that book so much, for reasons I will leave for another time.
I would pick ... Okumura's "Realizing Genjokoan:
Perhaps not a traditional 'Zen' book, but I greatly enjoyed The Man Who Folded Himself by David Gerrold. It's not without its flaws, but it explores
**cleaning teeth with a cypress toothpick**
A kindle self-destruct app.