Destiny of the Republic is about the slow, drawn-out, unnecessary death of President James A. Garfield. Garfield was officially killed by a man named Charles Guiteau, who had first convinced himself he was of major importance and should surely be awarded the ambassadorship of Paris, and then, when this didn't happen (because he had absolutely no qualifications or connections, regardless of what he thought), decided Garfield needed to go. Garfield was, after all try to do away with the cronyism that saw important civic posts handed out to lackeys and major campaign contributors, and actually get people selected based on merit.
As it stands, Garfield would likely have survived Guiteau's attempt if not for the attending doctors. Millard weaves into Garfield's story the resistance by the established American doctors to Joseph Lister's ideas about trying to create an antiseptic environment, to avoid getting germs in wounds. The idea had taken hold in England, but for whatever reason, most American doctors in the 1870s thought it was hokum. Which is how you get a situation where one doctor sticks his unwashed finger into Garfield's bullet wound, looking for said bullet, while Garfield is still laying on the 19th century train station bathroom floor. In reality, Garfield died because his primary doctor, a Dr. Bliss, got him riddled with infection and was too dumb and stubborn to realize it.
Millard brings in several variously connected threads and makes an engaging read of them. The early lives of both Garfield and Guiteau, and the distinct contrasts between them, but also the issue of Roscoe Conkling, a major adversary of Garfield, and the man who was certain he owned Garfield's Vice President, Chester Arthur. Arthur's transformation, thanks is large part to a series of letters from a Julia Sand, is detailed, and so is Alexander Graham Bell's involvement in the attempt to save Garfield's life. It's impressively done, and highly recommended.
'Not only did many American doctors not believe in germs, they took pride in the particular brand of filth that defined their profession. They spoke fondly of the "good old surgical stink" that pervaded their hospitals and operating rooms, and they resisted making too many concessions even to basic hygiene.'