So I ran into this problem tonight.
When clicking on define after selecting a word, instead of being taken to the customary brownish screen of the system dictionary, I got a blank grey screen that said 'no definition found.' There was an Edit button in the top right corner. Clicking on that took me to a long list of dictionaries: the revised American Oxford, the Oxford, the Apple dictionary, foreign language dictionaries, etc. They all had cloud download icons next to them.
I tried clicking on some of these but nothing happened. After a 10 second delay, I noticed the Apple dictionary had started downloading but it never finished. The circle stayed stuck at about 99%. The app then froze.
Rebooting took me back to my book, but attempting to define a word now causes it to freeze on this screen :
http://i.imgur.com/8suIuMv.jpg. Leaving the app and re-entering it causes Marvin to crash, and once even locked up my entire phone for 20 seconds.