Well, no, it's not. The adjectival form of "missionary" is "missionary." I might go so far as to say the word "missional" can never be properly applied, because it's not a real word. I mean, I'm definitely a descriptivist when it comes to English grammar and usage, but really, what's the point of using "missional" in place of "missionary?"
Semantics aside, the book is thought-provoking, though part of me feels like the authors may be overstating their case somewhat. There's no question that churches need to be more than "vendors of religious goods and services," as they quote theologian George Hunsberger as saying, and that there should be more to church membership than going to worship on Sunday and taking communion and so on, but my impression from the first two chapters -- which, admittedly, is all I've read so far -- is that they downplay the importance of the church as a place for community-building and spiritual formation. But like I said, I'm only two chapters in, so it's unfair to draw too many conclusions about it at this stage.