Guerilla Learning is not really an unschooling book. It's more of a book that describes how families with children still in school for whatever reason can still get some of the benefits of unschooling.
If you want to know *why* to unschool: John Holt (esp. Teach Your Own), John Taylor Gatto (esp. Dumbing Us Down) , David Guterson - Family Matters: Why Homeschooling Makes Sense.
If you want to know *how* to unschool: The Unschooling Handbook by Mary Griffiths will get you started. If you want to know how to unschool your preteen/teen: The Teenage Liberation Handbook by Grace Llewellyn.
If you want to begin to unschool yourself: The Teenage Liberation Handbook by Grace Llewellyn.
One of the best places to learn why and how to unschool? This forum.
One more thing. When I first stated homeschooling, I ran out and bought books. Later, I checked the education and parenting sections of our public library. Guess what? They had Llewellyn (even Guerilla Learning), Holt, Gatto, Guterson, and many others.
If you haven't noticed by now, I am a bibliophile. I have read many other books on homeschooling - several of which are great - but the ones listed above are fairly essential reading in my book LOL!