There are lot of books out there about how Christians should (or should not) engage in politics, but I’ve had three popular-level works on my to-read list because they pretty well cover the landscape: Jim Wallis’ God’s Politics , Wayne Grudem’s Politics – According to the Bible , and this one by Greg Boyd.
Doug Wilson blogged through Boyd’s book a few years ago. I read his posts immediately after Boyd’s chapters, and his commentary was extremely helpful (http://dougwils.com/tag/c125-greg-boy…; the posts are in reverse order). Wilson ably criticizes Boyd’s theology of political engagement, and calls it out for being a jumble of incoherence and inconsistency.
James K. A.Smith also reviewed the book for Christianity Today a few years ago (http://www.christianitytoday.com/ct/2…), arguing that Boyd’s view is simply pietism and pacifism re-warmed. Boyd makes a few good points about the excesses of modern conservative evangelicalism’s relationship to politics, but there’s little worthwhile content otherwise. Between Smith and Wilson you can learn all you need to about this book.