The Mislaid Magician or Ten Years After by Patricia C. Wrede and Carolyn Stevermer

In this sequel to The Grand Tour, it's been 10 years since their honeymoon and now Kate and Cecy have growing families. Soon they and their families are embroiled in a magical mystery. A
magician working for the railroad has seemingly disappeared into thin air and they must find out what happened.

This time around we get letters from James and Thomas as well.

Highly Recommended