Planets in Peril : A Critical Study of C.S. Lewis's Ransom Trilogy, David C. Downing. I picked this up for a research paper on Lewis, and while it's definitely academic, it should appeal to lay readers with more than a passing interest in Lewis as well.  Each of Downing's chapter's takes a different perspective on elements of the trilogy, from the portrayal of evil to its medieval influences. Excellent scholarship.

Originally read March 2005