Analyzing C.S. Lewis's whole body of work

Everyone knows C.S. Lewis as the writer of the Chronicles of Narnia series, as well as (to a lesser extent) The Screwtape Letters. However, he's written a lot of other works too–both fiction and nonfiction–that don't get as much attention. Throughout his body of work, what recurring patterns and themes emerge? How have they changed over time? Is there anything he wrote that doesn't get the attention it deserves?