So far, I like this. It feels so odd to intentionally leave a book somewhere, but it's for a good cause. It can be fun, too - like letterboxing and geocaching, the goal is to make the drop without letting anyone see you. At least, that's what's going through my head.
Anyway, if you have extra books, give it a try.
Incidentally, I did not buy the starter pack (why buy when you can do it yourself?). Instead, I printed out their label template [62kb, pdf] (for inside the cover) and also created another template [5kb, pdf] for the outside of the book cover. And instead of printing these on sticky labels, I just attached them with packing tape. Works just fine, and cost nothing.