I wanted to recommend Zen Questions: Zazen, Dogen, and the Spirit of Creative Inquiry by Taigen Dan Leighton. It's based on a series of articles and Dharma talks by Leighton and covers the practice of Zazen, and commentaries on Dogen's Shobogenzo and Dogen's Extensive Record. I've only just started reading but think it would certainly be a good fit for the book discussion group.