I think the problem with finding tips on "how" to unschool, on what resources to use, is that unschooling is by its very nature led by the child and different every time it's practiced. A sandbox and a classic novel might be what Kid A needs, while Kid B might need DVDs about ancient Egyptian mythology and Kid C might need a carpentry set or a border collie.
I do know of a couple of books that are about supplementing public schooling (or any educational situation, really) with an unschooling-type philosophy.Coloring Outside the Lines: Raising a Smarter Kid by Breaking All the Rules
by Roger Schank
Schank, the founder and director of Northwestern University's Institute for the Learning Sciences, is admittedly a little eccentric and largely anti-school in his opinions. That turns some readers off, but it sounds like the book has many thought-provoking suggestions anyway. From an Amazon.com review:
"Roger Schank doesn't accuse teachers of trying to squelch children's interests or administrators of being bad people, but he does point out that the way the school system teaches is completely outdated and unintentionally destroys children's eagerness and passion for learning. To raise a truly intelligent child, Schank says, parents must take charge personally. They must work to undue [sic] the damage school does to a child, and to instill positive character traits in a child that will help him develop true intelligence: verbal ability, analytical ability, gumption, inquisitiveness, creativity, and ambition. There are simple (and not-so-simple) ways parents can do this, and Schank dedicates his book to telling us how we can help our children and also WHY we should take charge. "Guerilla Learning: How to Give Your Kids a Real Education With or Without School
by Grace Llewellyn and Amy Silver
"GUERRILLA LEARNING IS CREATING A HOME ENVIRONMENT THAT FILLS YOUR CHILD WITH THE JOY OF LEARNING
"Let your daughter read her library books instead of finishing her homework . Ask your eleven-year-old’s beloved third grade teacher to comment on his poetry. Invite a massage therapist to dinner because your daughter wants to go to massage school instead of college. Give your child the freedom to pursue his interests, develop her strengths, cultivate self-discipline, and discover the joy of learning throughout life.
"If you’ve ever felt that your child wasn’t flourishing in school or simply needs something the professionals aren’t supplying, you’re ready to become a "guerrilla educator." Revolutionary and inspiring, Guerrilla Learning explains what’s wrong (and what’s useful) about our traditional schools and shows you how to take charge of your family’s education to raise thinking, creative young people despite the constraints of traditional schooling.
"Filled with fun and exciting exercises and projects to do with children of all ages, this remarkable approach to childhood, education, and life will help you release your child’s innate abilities and empower him or her in the wider world that awaits beyond the school walls. "
Hope that helps!