Lord Kalvan

Lord Kalvan of Otherwhen (with spoilers) is a classic parallel universe story by H Beam Piper. A State Police corporal, Calvin Morrison, is transported to an alternate North America with an early Renaissance level of technology. He gets involved in a war between two princes and uses his knowledge of high school chemistry (which you wouldn't get in high school now--I took chemistry and they completely omitted the "gunpowder" part) and military history and tactics to alter the course of the war. It's a really enjoyable book, and the only downside is that it ought to have a map included. There are several editions available on Amazon; there's at least one wargame rules set based on it, and I've seen a Diplomacy variant and several miniatures battles based on it.