One short (144 pages)
Carl R. Trueman. Republocrat: Confessions of a Liberal Conservative. Phillipsburg, NJ: P&R Publishing, 2010. 
(Sample pages, video interview, and endorsements here.)
One long (624 pages)
Wayne Grudem. Politics according to the Bible: A Comprehensive Resource for Understanding Modern Political Issues in Light of Scripture. Grand Rapids: Zondervan, 2010.
Southern Seminary’s “Towers” will publish an extended interview w/ Dr. Grudem along w/ a review of his new book in its first September issue. Look for it!
I had the privilege of hearing Dr. Grudem present an outline of the book last summer. It looks to be excellent!
From what I know of these two, their books will probably come to fairly different conclusions. Dr. Grudem sees a closer tie between doctrine and ethics. For example, many leftist Christians are egalitarian and say that this is truly Christian. Dr. Grudem shows how this denial of the Authority of Scripture inevitably leads to other denials of Scripture (homosexual practice etc.).
I think an example of the difference can be seen in John Frame’s devastating critique of Michael Horton (who endorses Dr. Trueman’s book). It can be founds here: http://www.frame-poythress.org/frame_articles/2006InDefense.html
Trueman and Horton in the end, seem to be calling for withdrawal from the political realm in reaction to a prevalent type of cultural “evangelicalism” that is political but not theological. Unfortunately, their books will never make it into the hands of these “evangelicals”. So their books end up being a call for withdrawal from loving your neighbor through this area of culture.
Grudem and Frame are doctrinal first, but understand that loving your neighbor in the political realm is one of many necessary ways Christians love their neighbors. I can’t say it nearly as well as John Frame’s article.
This morning, I posted on Dr. Moore’s critique of the Glenn Beck rally and the hopes that Dr. Grudem’s book will provoke a thoughtful, prolonged discussion on engaging public policy issues from the pulpit.
I really think seminaries need to step up and help out here!
Check out an exclusive pre-publication book review of Grudem’s book at the Jubilee Centre website.