Jive (Lightning Riders, #3)

Time agent Jewel Jamison promised a young Jack Cavanaugh that she'd be his 26th birthday present. She'd hoped for a fling with the man she'd become obsessed with in the history books, but Jack, having waited ten years for the sexy Jewel to show up in 1919, has plans to build a life together.
Before they can even begin to explore the possibilities, however, Jewel realizes that the timeline is now missing something -- namely World War I. A full-scale "time crisis" is raging as two factions battle through history--one trying to erase war and the other trying to erase time travel and all traces of it. Including traces like Jack Cavanaugh.
As the "Time War" rages, Jewel has to choose: keep her promise to Jack and stay out of the war or break her promise in order to save him.