As a child, I couldn’t understand why my parents always took biographies of French presidents or memoirs by mountaineers on holiday when a sensible traveler would surely prefer “Evil Under the Sun” by Agatha Christie. Now I get it. Forget beach reads; vacations are the best time to bite off a serious hunk of history. Here are four new books that offer something to think about as you wait for your plane, your train or the end of the summer rain.