McClellan, Sherman & Grant, by Thomas Harry Williams
Pretty much anything by P. J. O'Rourke. Most of his books can be had used on Amazon for under $5 shipped.
If you want to loose all faith in humanity, read
Ordinary Men: Reserve Police Battalion 101 and the Final Solution in Poland
by Christopher R. Browning
In short, it's a posthole book about some voluntary participants in a particular series of mass killings during WWII.
I Cannot Forgive by Rudolf Vrba - also a Holocaust book. Details being rounded up, shipped off, camp life, escape, telling the world.
Turning Points of the Civil War
by James A. Rawley
The Last of the Fathers : James Madison and the Republican Legacy
by Drew R. McCoy - particularly well done. I'm somewhat biased, though, having studied under him.
The Gulag Archipelago by Aleksandr I. Solzhenitsyn - how Stalin's USSR worked to isolate every person
La Place De LA Concorde Suisse
by John McPhee - sort of a fluff book about the Swiss military. Author participates in some exercises. Unabashed adoration. Still, it's an entertaining read.
Founding Brothers: The Revolutionary Generation
by Joseph J. Ellis
The Future of Freedom: Illiberal Democracy at Home and Abroad
by Fareed Zakaria
-- On why a rush to elections in newly free countries is often a Bad Thing. Tremendously readable political work with good insight into the world situation.