There are the nice cards, professionally made, as a physical product. That is the “book” part of the book/ebook bundle. They are unbound (loose like any other deck of cards). They’re also available in five-packs and ten-packs.
The physical cards are the most convenient format as far as I’m concerned. You can pin them to walls or sticky-tack them to the edges of your monitor, and whip them out at meetings or share them with friends, sort and organize them as you like, use them as “magic 8 ball of Agile” by shuffling and randomly selecting, etc. You can write on them, stick notes to them, underline and highlight good parts as you choose. They’re handy.
Then there’s the e-book which is appropriate for your phone, kindle, nook, netbook, or whatever. It’s great for reading on the train/plane/whatever, and can be projected up on a screen during meetings, but lacks the tactile advantages of the physical deck. I think you can print out personal copies. I’ve printed personal copies here at home, but I don’t like using them because they’re flimsy, and a pain to assemble front-and-back and cut to size.
I like using the digital form digitally, and the physical form physically. The ebook/book bundle gives you both.