This 2009 Baker Publishing book by Alan Roxburgh and Scott Boren provides an introduction to missional thinking in terms which the average person can understand. Basically, there is no set definition of "missional." And there is no missional model. But there is a missional river that involves mystery, memory, and mission. This river transcends denominational labels and traditions.
God is at work in our communities and churches become active participants in that mission, not through externally imported programs designed to attract people, but through a process of listening, engaging, and risking. It is a matter of learning the context (as any missionary would do when entering a foreign culture) and then as a church discerning how God is present and at work in the context.
The first two-thirds of the book are about the development of a missional mindset and how the church becomes a "contrast society" in a postmodern and post-Christian culture. In the last third of the book Roxburgh and Boren describe the discovery process through which they lead congregations that are wanting to change.
This is not really a "how-to" book (in spite of the subtitle) and it's certainly not a new program or trend. Many churches will find that they're already swimming in the river -- at least some of the time. Even struggling congregations (and their leaders) will find some encouragement here.
The Spirit of God is among the people of God, not in the experts and the ordained, but right in the midst of all the ordinary men and women of the local church. This means that God's very Spirit is in the local church, among the ordinary men, women, and children who comprise this faith community. Therefore God's imagination and God's future is among the ordinary local men and women who gather to worship, confess their confusion, who know the church is messed up and needs to change, and who confess that they don't have a clue what might be involved in this change or how to go about it. This is where the Spirit is at work... And therefore, the job of the leadership is not to come up with grand plans for the congregation but to cultivate an environment in which the missional imagination of the people of God is called forth in live action and lives. This happens as we create spaces and time for people to trust their own voice as a people, to dwell together in the Word, to listen one another into free speech, and to dare to ask in dialogue with one another what the Spirit might want them to risk in connecting with their neighborhoods and communities. ~ p. 165