This bug has been resolved in v1.51
There is currently a problem decoding some books for Spritz.
We've found and corrected this bug, but we can't push an update until Dec 29th since Apple don't accept updates during the holidays.
If you experience this problem, you can send the offending book to us at firstname.lastname@example.org and we will manually edit it for you so that it will work. Please note that this is a temporal fix and a proper update will come as soon as we can upload the update to the Apple servers.