Along the lines of Poul Anderson's "Time Patrol" stories, one I recommend is _Days of Cain_ by J. R. Dunn.
The protagonist is Monitor Gaspar James, an agent of the Moiety. Anderson's Time Patrol is the creation of the Danellians - far future descendants of humanity. The Moiety is
really far future. The intelligences that run it are plasmoids existing in the era of the heat death of the universe, when stars and planets are no more, drawing energy from the slow process of universal cooling.
The advanced technology of the far future galactic era made time travel possible, and Gaspar and his fellows labor to prevent changes in history caused by various folks who would prefer to see things come out differently than they actually have. He suffers a crisis of conscience when he is assigned to stop a rogue Moiety agent who has decided she will alter history, and intends to use her knowledge and weaponry from the future to prevent the Holocaust.
The experience forces Gaspar to re-examine his beliefs about the job he does and the motives of those he serves.
It's a science fiction examination of the theological "the problem of pain" - if there is a just, merciful, and loving God, why does He permit atrocities to occur? The Moiety
could alter history and prevent the Holocaust (or other great atrocities of history). Why don't they? They have reasons, but all may not agree with them.
I don't believe it's available as an ebook, but Amazon has paper copies:
http://www.amazon.com/Days-Cain-J-R-Dunn/dp/038079280X
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Dennis