Okay, I deleted 2 of the books (ironically, both Kindle How-To guides), but was unable to find the other, ASIN# B001CBBTJM. Tried updating, it was unsuccessful once again. Switched to debug log mode, and have attached the most recent update logs.
I, too, feel like the hacks are not the cause of the problem - otherwise, why would the previous update have worked? I don't have to have this 2.5.6 update, but I would like to know a way to prevent it from downloading and trying to install each time I turn on Whispernet and/or sleep mode.