Connie Willis has already been mentioned. Doomsday Book was especially poignant, as people were dying of the plague in front of the protagonist, who knew there was a simple cure in her own time.
Also, Harry Harrison's A Rebel in Time. No perfect, but teenage me enjoyed it immensely. A time and place I have little investment in, or great knowledge of, as I'm not from the US, but it brought it to life for me. It also provided some details I knew nothing of previously, and it was a rip-roaring adventure.