The Guardians of Time, by Poul Anderson

I am not a fan of time travel stories. Poul Anderson's Time Patrol stories are the best of the whole silly subgenre.

Time Patrol: Manson Everard is recruited by the Time Patrol, and goes back to fifth century England to stop a 30th century merchant from changing history.
Brave to be a King: Everard tries to rescue a friend who is stranded in history and has been forced to play the historical role of King Cyrus.
Gibraltar Falls: A fact-finding mission to the formation of the Mediterranean sea almost ends in disaster.
The Only Game in Town: Everard and Sandoval head to pre-Columbus America to sabotage a Mongol expedition to the Americas.
Delenda Est: When the timeline is changed radically, Everard and van Sarawak must locate the critical event that was changed--which turns out to be related to Hannibal--and fix it.
Of Time and the Rover by Sandra Miesel: A brief essay about the celebration of individualism in Poul Anderson's Time Patrol stories.