John Brunner's Times Without Number. Three "Society of Time" novelettes, starring junior officer Don Miguel Navarro. Four centuries had passed since the Spanish Armada's glorious defeat of England, and the subjects of the Spanish Empire were, by and large, proud to be in it. The men of the Society of Time were dedicated to keeping things that way. But corrupt officers of the Society were using time travel to make a little profit on the side, and the Indians of the New World longed to be masters of their own lands. In the second story, somehow, Amazons had come through time to assassinate the Imperial Prince. And in the third tale, Don Miguel faced a threat to all of history...

Spoil of Yesterday
The Word Not Written
The Fullness of Time

These 3 short novels (originally from the British magazine Science Fiction Adventures in 1962) were first collected in 1962 as half of an Ace Double, then revised and polished in 1969 for the single paperback edition (this final version has been reprinted by several publishers.) One of my favorite books by Brunner, and I strongly recommend it.