The Impeachers by Brenda Wineapple

The impeachment and trial of Andrew Johnson represented yet another attempt to preserve the Union and free the slaves, which, to the impeachers, were the self-same thing: to preserve the Union meant creating a more perfect one, liberated at last from the noxious and lingering effects of an appalling institution that retreated human beings as property.
-- Brenda Wineapple, The Impeachers: The Trial of Andrew Johnson and the Dream of a Just Nation (Random House, 2019)

One thing we have learned from all three impeachments of America presidents is, as Wineapple writes, "sleaziness is hard to nail down" and what actions call clearly for impeachment is subjective and based on party affiliation as much as anything. The Johnson impeachment failed the necessary two-thirds vote by one vote, as all Democrats voted against conviction, along with seven Republican senators.  The seven Republican senators had various reasons for their votes; some were perhaps bought off (though this was never proven), but mostly: they did not want Radical Republican Benjamin Wade (president pro tempore of the senate; there was no vice-president) to become president, they were afraid to make Johnson a martyr, and they were concerned that conviction might keep Ulysses S. Grant from becoming president.  Johnson had many shortcomings, of course, especially his desire to return the southern states to the union quickly on dubious grounds and his hostility to allowing former slaves to vote, but whether his violation of Congress's tenure of office act, when he fired Edwin Stanton as Secretary of War, was sufficient grounds for impeachment remains controversial.

Wineapple's attention to the details of the case and the dramatis personae is impressive. I would have preferred fewer details of some of the more marginal characters and could have done without Wineapple's subjective descriptions of character, e.g., Benjamin Wade was "burly and belligerant, profane and tenacious," and more details of their actions.  The comings and goings of so many characters is slightly confusing and I would have preferred somewhat more historical context, but Wineapple's research and writing is impressive.