The Mislaid Magician or Ten Years After — Patricia C. Wrede; Caroline Stevermer

Ten years have passed since Kate and Cecy married Thomas and James, and England is now being transformed by the first railways. When James is asked to look into the sudden disappearance of a German railway engineer, he and Cecy make a shocking discovery: The railway lines are wreaking havoc with ancient underground magic, which could endanger the very unity of England. Written in letters between Kate and Cecy--and between their husbands--this installment of the cousins' adventures is another satisfying blend of magic, mystery, adventure, humor, and romance.