I did a lot of research on this for a book, and this is the best site on the internet about it:
Squalor Survivors
It's the stories of recovered hoarders, what led them down that path, and how they got better, along with articles about squalor conditions. Plus pictures!
I have much more sympathy for these people now--it's not laziness or character failings, it's the outward expression of serious mental issues.
Also, they reference David Allen and GTD several times.