One book on my mental must-have list is The Velvet Room by Zilpha Keatley Snyder. It's perfect for making a child's imagination soar.
Another one that I don't think has been mentioned yet is My Side of the Mountain by Jean Craighead George. Great story about a young boy learning to live off the land. Also Julie of the Wolves by the same author. I haven't read the sequels so I cannot comment on them.
Island of the Blue Dolphins by Scott O'Dell is another one I would want to have on hand. Excellent story of survival, a woman living alone on an island several years before being rescued, and based on a true story, too.
There are others that aren't coming to mind right now, and of course many of the ones already mentioned, such as the Bible, Box Car Children, Chronicles of Narnia, Little House series, etc.