Wow, this is
insane. I have experienced a similar issue with Google Books on my Nexus 5, so unfortunately it looks like this may be an issue with how the operating system is handling the background data for e-reader apps.
I just recently switched to Ting (MVNO on Sprint's network) and was looking forward to a $30 bill this month, but Google Books said "Nope, I'm going to use 735 Megabytes of data to do absolutely nothing". In only a few minutes of reading plain-text pdfs I used 690 MB of background data and 45 MB of foreground, and that's without downloading ANYTHING. Even with ONLY the foreground I could download the entire 1000 page PDF three times over with room to spare, and that's with NO compression!!! I understand there will be some overhead, but that should be dwarfed by the savings from compression.
Google has cost me over $15 in data charges for absolutely nothing.
I don't understand how so much data is used. I could download that book 50 times in 700 MB of data, yet I have nothing to show for it. I have contacted Google about this, and now that I've found this and other reports of the Kindle reader having the same problem I know it is something wrong with the app or with Android. If I keep getting the "do x, y, and z to install the book to your phone" runaround I will end up demanding some Play Store credit to make up for the money their software is wasting.