Robert Adams is probably best known for his post-apocalyptic Horseclans novels, but in keeping with what has turned into a December Marathon of Time-Travel Fiction, I re-read his Castaways in Time.
Unfortunately, the Kindle version of this classic is full of OCR typos, which lost it a star, but the story is a good one. A mixed group of travelers battling an epic flood on the East Coast of the United States in present-day is suddenly (in a manner never explained) transported through time, space, and a "lateral" time dimension to a "world of alternate probability". They seem to be in 1640 England, landed into the middle of a Roman Church-supported war between the Scots and various Crusaders, against the heretical English king. Not Henry VIII. Arthur III.
The displaced persons immediately set about using the goods that were transported with them—a truck-load of high-nitrogen fertilizer, a cabinet of modern and older firearms, along with various other goodies (such as bottles of Jim Beam, penicillin and vitamin tablets)—to carve themselves names and reputations in the new world, supporting good King Arthur against a Roman Church that controls the world through its monopoly of the known component of gunpowder, niter.
The action is wholly devoted to the ways in which advanced knowledge of warfare and its technology is used within the medieval armies to alter the balance of power. In that, it reminds me of similar "time travel" novels such as H. Beam Piper's Lord Kalvan of Otherwhen (sadly not available on Kindle, Lord Kalvan launched the "alternate history" and "time police" concepts for such works), and Mark Twain's A Connecticut Yankee in King Arthur's Court, which primarily used the contrast of modern and ancient technology to poke fun at current-day society.
Adam's purpose is not so lofty as Twain's, nor as well-envisioned as Piper's, but the way he presents warfare of the time with all its "mud and blood", and the masterful way he describes the problems of a feudal society coping with the deprivations of war and weather, makes this a worthwhile read.
Just put your patience on high if you are disturbed by typos. Combined with their plenitude is a liberal sprinkling of Gaelic and German phrases that make reading the novel on Kindle a little more difficult than it should be. Better yet, buy one of the many used paperback versions, and enjoy it in its original well-edited text.