Discussions about digital migration are often ham-strung by a version of the either/or fallacy. I'm sure the boss of Waterstone's would love to believe that Kindle sales have all but disappeared and the paper book is making a comeback. I'm sure it's not as simple as that.
The habit matters more than the product and the Kindle may have re-awakened the habit of reading books among some people. If I'm reading something major nowadays, the kind of thing that I'm going to want to have in my pocket to read at the bus stop, I'll tend to buy a cheap paperback and also get it on Kindle. Generally speaking, books are cheap. The major investment is the time devoted to reading them. If I buy both forms I'll read more quickly, which is a more satisfactory way to read. I'll then get through more books, which will make me feel better and also look for more books.
I think that's what they call win-win.