Fridge: This Book Is Full Of Spiders Seriously Dude
David wants to keep his hometown a secret in the first book, so he refers to it as Undisclosed. It doesn't make any sense for him to continue doing this in the sequel because the events he describes are national news. Everyone would already know what town he's talking about.
The epilogue explains how it killed maybe 70 people in a small town and, in the end, no one cared. It's possible the whole event was later erased, leaving behind Wong's memoir of the time, the same way David's toolshed John Dies at the End contained magazines depicting events (like Bill Clinton's assassination) that were expunged from reality, despite the fact that such expunging would logically make the magazines no longer exist.
If Amy retroactively loses her hand near the end of this book, then how did she open the "phantom limb" door and latch in John Dies at the End and in this book?
Well, that's just it, isn't it? It's retroactive, so it's always been that way. Cause and effect don't work in a way we can really understand with them.
Fridge Brilliance / Fridge Horror: A recurring theme of the book is whether or not the infected will inevitably "monster out." Although they never explicitly say it, many of the major characters know that at the end of John Dies at the End, David was been replaced by a different kind of monster who hasn't "monstered out" yet.
The therapist discusses the Fridge Horror in Star Trek that the teleporter arguably kills everyone who uses it and replaces them with a doppelganger.
After the title of the first book didn't happen, it was cool to realize this one actually was full of spiders.