I also own the book (that being said I first bought the book and later found out there’s an app). The biggest difference between the book and the app is that the book solely uses katakana and hiragana for the readings. As I mentioned in earlier blog posts, I’m normally a person who loves to highlight and write notes in textbooks. However, this time it’s different. I find the book to heavy to carry around and finding a kanji takes up a lot of time. The book is also a lot more expensive (it was around 40 € last time I checked on amazon) whereas the app was just 18 €. And the app is much more useful as it can also be used offline and it’s virtually always with you. So studying on the bus etc. is really comfortable with the app.