I would have to say the one I found most interesting in terms of theology and theory was "Think Orange" by Reggie Joiner. The exposition of Deuteronomy 6 alone makes the book worth it, but there is whole lot more in there than that!
Practically, the one that challenged me the most personally has been "The Fabulous Reinvention of Sunday School" by Aaron Reynolds. The first time I picked it up in the store, I glanced at the first chapter about memorizing your teaching, and I put the book back down immediately. It obviously wasn't for me. :) Anyhow, I later bought it and read it, and it's full of great stuff! And, yes, I do memorize my lessons now, and it has been worth it!
What is the best / most impactful thing you've read recently?
