Dark Destiny — M.J. Putney

Tory and her friends are celebrating the success of a second mission to aid WWII Britain when an urgent summons calls them back to their own time: Napoleon is on the brink of invading England. Can a handful of young mages stop an army? In desperation, Merlin's Irregulars ask Rebecca Weiss, an untrained telepath from 1940, to come back to 1804 and change Napoleon's mind before it's too late. As Tory and Allarde make a commitment that will cost him his inheritance, Rebecca promises to do what she can to stop Napoleon even though she is unsure of her magical abilities. But Tory and her friends saved the Weiss family from Nazi imprisonment, and Rebecca will risk anything, even her life and her budding relationship with Nick Rainford, to repay them. A daring mission takes Tory and Rebecca and their friends into the heart of the Army of Boulogne, where Napoleon is plotting invasion. But while their success may save England, they must still face the bittersweet consequences of their decisions when they return home.