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.