The Forgotten Ways is THE book on missional/emerging church If you want a complete explanation of what the emerging church really is about, this is the place to look. Here the core issues are discussed in an excellent way, in what he calls "apostolic genius" (lame title, if you ask me), which is described as "the centrality and lordship of Jesus, disciple making, the missional-incarnational impulse, organic systems, apostolic environment, and communitas (a type of community formed in situations of significant ordeal and/or mission)."
I didn't learn much in this book, to be honest. But it is a great way of collecting all theological thinking I (and many others) are assimilating for some time now. This is the kind of book to offer if you lack words to explain what missional church is all about.
On the down side, the book is a little hard to read. It's not the language, it's just that I wished the author was a better writer. But it's an excellent technical/theological book, he's not trying to make a novel here. What he achieves in this book is a very good systematization of missional church theological thinking, without lacking the practical aspect.
For all this, it's a must buy. Buy it by the dozens, and offer it on Christmas and birthdays.