The Rise and Fall of the Third Reich, William L. Shirer (A slow read as it is full of history and long footnotes, but worth it).

The Rise and Fall of the Third Reich is a non-fiction book by William L. Shirer chronicling the general history of Nazi Germany from 1932 to 1945. It was first published in 1960, by Simon & Schuster in the United States, where it won a National Book Award.[1] It was a bestseller in both the U.S. and Europe, and a critical success outside Germany, where harsh criticism stimulated sales. Academic historians were generally critical.
Rise and Fall is based upon captured Third Reich documents, the available diaries of propaganda minister Joseph Goebbels, General Franz Halder, and of the Italian Foreign Minister Galeazzo Ciano, evidence and testimony from the Nuremberg trials, British Foreign Office reports, and the author's recollection of six years reporting on the Third Reich for newspapers, the United Press International (UPI), and CBS Radio —terminated by Nazi Party censorship in 1940.
to be followed by Up the Line, Robert Silverberg

The story's protagonist is Jud Elliott III, a failed Harvard history masters student in 2059. Bored with his job as a law clerk, he takes up a position with the Time Service as a Time Courier.After an introductory course, Jud shunts up and down the time line ("up the line" is travel into the past; "down the line" is forward time travel, but only to "now-time," Jud's present of 2059) as a guide for tourists visiting ancient and medieval Byzantium/Constantinople.
Jud's problems include not only stupid tourists, but also greedy and mentally unstable colleagues who attempt to cause various types of havoc with the past. He is forced to break the rules in order to patch things up without drawing the attention of the Time Patrol.