The information below was true in 2013, and it's still interesting, but the series is now a free e-book. If you subscribe to Pam Laricchia's e-mail newsletter, you can download a copy of Exploring Unschooling
Pam: It starts with a series of ten emails I wrote, titled "Exploring Unschooling". The emails are delivered two a week over the course of five weeks. I was trying to accomplish the feeling of walking hand-in-hand with the reader as they learn about unschooling; I basically imagined how the conversations might go if I was meeting a new and interested person for coffee on a regular basis to chat.
I like it as an intro, and that it's spread out over time. It encourages them to question and think along the way, to focus on each new piece of the puzzle for a bit before moving on to the next one. I think it stands nicely apart from both books, while still weaving in ideas from both. Over the course of the series there's a couple links to your site, a quote from you ("Read a little, try a little, wait a while, watch."), and attribution for some ideas (like rules vs principles). I had a lot of fun creating it. :-)