As a one-day-at-a-time recovering mid-level hoarder...I highly recommend the book
Buried in Treasures, by Tolin, Frost and Steketee.
Many libraries have it. Here is a link for it at Amazon:
http://www.amazon.com/Buried-Treasur...p/0195300580#_
Note that you can take a peek inside the book to get a flavor for it.
There are many root causes and different types of hoarding. This book is so very helpful for hoarders and their families in dealing with this illness in a practical way and, most importantly, in one that will not increase the resistance of the hoarder.