Dark Passage by M. J. Putney. I really enjoy Putney's adult novels and also really liked Dark Mirror when I had a chance to read it.
Here is the product description:
The Irregulars return home to 1803 England safely, but their worldview has changed. Not only have their heroic efforts at Dunkirk given them pride and confidence but their dangerous mission has increased their magical powers.This sounds like it will be another entertaining story with magic, time travel and romance. I can't wait to get it. Come on, big brown truck! Show up at my house, please.
Tory delights in the ever deepening bond she shares with Allarde until she discovers how powerfully he is connected to his ancient family estate—the lands he will not inherit unless he denies his magical powers and chooses a nonmagical mate. If Tory really loves him, she must walk away—but does she have the strength to leave the love of her life?
Cynthia’s heroic efforts at Dunkirk have won her the respect of the Irregulars, but her sharp tongue keeps everyone at a distance. Isolated and very alone at Lackland Abbey over the Christmas holidays, she reluctantly agrees to join Jack Rainford and his family for their celebration even though they’re commoners, far below her own noble rank. The warm welcome of the Rainfords makes her feel happier and more accepted than she has ever been. But she can’t possibly be falling in love with flirtatious Jack! Can she?
Then the Irregulars are drawn into a dangerous attempt to rescue a vitally important French scientist from Nazi-occupied France. Tory and Allarde must work together because countless lives are at stake. Disaster strikes and not only is their mission threatened, but their very lives. Can magic and their loyalty to each other help them survive to return home?