While many of the letters featured on the app also appear in the book, the app also includes some exclusive goodies of its own. Of the 78 letters included, 10 of them are exclusive to the app. What's more, there is audio commentary from Davies, audio readings of the letters, and — perhaps best of all — a foreword from Lennon's widow, Yoko Ono.
Digital Music News notes that this will most likely be exclusive to iOS (that is, Apple mobile devices), and that it will include a soundtrack of Beatles music.
A release date for the app hasn't yet been officially announced, but the official website notes that it will be "available soon." Meanwhile, you can order the regular book version of The John Lennon Letters from Amazon.