The newly-released The Unschooling Handbook:  How to Use the Whole World As Your Child's Classroom is a fantastic resource. It's by Mary Griffith. There is a review at http://iqcweb.com/fun/

Pretty much anything by John Holt  Including: Teach Your Own, Learning All the Time; How Children Learn, and How Children Fail

http://www.leapingfromthebox.com/art/subjectindex.html

A webpage with several articles about homeschooling and unschooling