Robert Strauss, Worst. President. Ever.: James Buchanan, the POTUS Rating Game, and the Legacy of the Least of the Lesser Presidents (Lyons Press, 2016)
The publication date explains how it was still possible to attribute this superlative to Buchanan, who sat there unmoving as the nation plunged towards civil war, having already endorsed the Dred Scott decision, which essentially negated the free states' anti-slavery laws.
The book starts out as whimsically as its title, with the author pawing through his father's history book collection for info on Buchanan - there isn't much. But it mutates into not just a full biography of the man, but a history of his political times. There's an entire chapter which has very little on Buchanan, but is a detailed account of the political background behind the presidential election of 1856. There's also a comparison of Buchanan with other Bad Presidents, explaining why he's worse.
The one thing Strauss doesn't do is explain how Buchanan could be such an active, even belligerent executive in other areas - the Mormon rebellion, the Pig War, he even launched an invasion of Paraguay (Paraguay?!) after the government there fired on a US ship - and yet be so inert at southern threats to take over US forts. Some of the other books on Buchanan that Strauss mentions - he's quite thorough bibliographically - get into this, but Strauss doesn't allude to it. (Basically, he was secretly sympathetic to their cause, but this motivation got buried in postwar historical analysis.)
Speaking of which, there's
Jonathan Horn, The Man Who Would Not Be Washington: Robert E. Lee's Civil War and His Decision That Changed American History (Scribner, 2015)
The decision was, of course, the one to resign his US Army commission and go with his state Virginia's decision to secede, even though he thought secession was a bad idea, had nothing but misgivings about the war to come, and he wasn't enthusiastic about slavery despite being a major slave-owner himself.
What Washington had done was declare himself primarily a citizen of the US as a whole. He would never, Horn says, have supported a state's secession. Why didn't Lee? Why did Lee decide that the one thing he couldn't do was raise arms against his native state?
Horn doesn't really answer this question, and despite the title doesn't concentrate on the decision. This is a full biography of Lee, though it skips lightly over some of the eventless years in the peacetime army. There's nevertheless a lot of interesting background, especially about his personal and family finances. Did you know that 1) Lee was available to lead the Harper's Ferry campaign against John Brown, although he was stationed out in Texas at the time, because he was home on leave after his father-in-law died? 2) that for many years Lee was not a legal resident of Virginia at all, but of the District of Columbia, because Arlington was in the west bank portion that was part of D.C. until retroceded to Virginia in 1846? 3) although he'd previously worn a mustache, he didn't grow his beard until after the Civil War started?
