This article in the LA Times says he’s reading about salt, Russian history, and the flu.

Salt: A World History by Mark Kurlansky
Alexander II: The Last Great Tsar by Edvard Radzinsky
The Great Influenza: The Epic Story of the Deadliest Plague in History by John M. Barry

It sounds like a good selection of books to me I might even add the salt book to The LIST. Oh, and I got the tip on Mr. Bush’s reading habits from H2Oboro lib blog, a good blog place for readers sent down via internet all the way from Maine.