Millard concentrates on Garfield himself, as well as Guiteau (framing his actions within the context of his very troubled life), Garfield's self-appointed chief doctor, Dr. Doctor Willard Bliss (yes, that was his name) and Alexander Graham Bell. What? Alexander Graham Bell, you say? Indeed, the inventor of the telephone played an important role in the attempted treatment of the wounded president (and, as you'll read, probably would have played a larger role if not for the aforementioned Dr. Bliss).
While I knew the basic outline of the Garfield assassination story before (that he was shot but lived for a while afterward), Millard does a great job of putting that period in context, explaining what was going on in the country at the time and how various folks reacted to the events. And she outlines in sometimes sickening detail how the medical "treatments" administered by Garfield's doctors did more harm than good, without a doubt hastening and probably even causing his death.
The book also cogently explains the serious rifts within the Republican party which led to Garfield's surprise nomination in 1880 (Millard's account of the convention alone would make this book worth a read) and to the intrapartisan strife and rivalries which plagued his short presidency (and which contributed in no small way to Guiteau's actions).
Highly recommended. Good popular history at its best.