Schlesinger’s letters complete a download that has been coming steadily since his death in 2007. Indeed, after going to his reward, he has been publishing at a prodigious pace. First came the Journal, in a hefty volume in 2007. Then, in 2011, the lengthy interviews he conducted in 1964 with Jacqueline Kennedy. Now, the letters, lovingly culled by his two sons, Andrew and Stephen, which offer more grist for a mill that was not exactly grist-deficient.

In a postscript, the editors recount the process of sifting through this pile of paper—134 boxes, with about 200 letters in each—and estimate that their father wrote 35,000 letters! Evidently, he never sent one without making a copy—ergo, this book. This paper trail seems almost incomprehensible in the Age of Twitter—letters, written on paper, composed of full sentences and paragraphs, making complex arguments, rooted in history and facts. Reading it during the government shutdown, it felt like an ancient cuneiform, testifying to the strengths and weaknesses of the civilization that preceded our own time. Ours feels smaller—tweetier—in comparison....