The Shield of Time — Poul Anderson

Manse Everard is a man with a mission. As an Unattached Agent of the Time Patrol, he's to go anyplace --and anytime!--where humanity's transcendent future is threatened by the alteration of the past. This is Manse's profession, and his burden: for how much suffering, throughout human history, can he bear to "preserve"?

Wanda Tamberley is a Patrol member in search of her mission. Recruited from sunny California in the late 20th century, she'd rather serve as a scientist in the research branch, exploring Earth's flora and fauna in epochs long past. But as hints accumulate from the Patrol's mysterious leaders uptime, It's beginning to look as If a lot of human history depends on her personal decisions--and Manse's.

Meanwhile, the Exaltationists are on the loose, determined to revise human history and rule Time forever ...and Manse Everard is sworn to stop them, no matter what the heartbreaking cost!