There's one book I can name that changed my life forever. Originally published in 1991, Don Aslett's Not For Packrats Only is a simple book that explains WHY clutter is such a bad thing and then gives tips on how to rid yourself of it. When I first read it as a teen, I had mounds of stuff under my bed. My closet was so full that I could barely close the door. Searching for something I hadn't used in a while took hours sometimes.
This book takes you room by room through your house, asking the question "What's the worst that will happen if I get rid of 'X'?" It's a refreshing read and helps you boil things down to their essence: is this thing helping me reach my life's goals?