Andrew Johnson, our 17th President, was eager to reunite the former Confederate states into the Union, but the Republican Congress impeached him for failing to protect former slaves. The first President to be impeached was saved from conviction by one vote in the Senate. The best general biography of the man is probably "Andrew Johnson: A Biography" by Hans Trefousse, though the Oxford-published "Andrew Johnson and Reconstruction" by Eric McKitrick also deserves a look.
Andrew Johnson succeeded President Abraham Lincoln in 1865 upon Lincoln's assassination. Political rivals of Lincoln tried to impeach President Johnson for carrying out Lincoln's plan to bring the South back into the Union. The House of Representatives voted to impeach, but the Senate failed by one vote.