The Mislaid Magician, or Ten Years After -- Patricia C. Wrede & Caroline Stevermer

Ten years after the events in The Grand Tour, Kate and Cecy and Thomas and James are still happily married. 

The first railways are being built in England and there is some concern that there may be magical side effects.  When a German magician/railway engineer disappears suddenly, the Duke of Wellington asks James to look into it.  As magic may be involved, Cecy goes with him -- and they leave all of their children with Thomas and Kate.

The Mislaid Magician or Ten Years After: Being the Private Correspondence Between Two Prominent Families Regarding a Scandal Touching the Highest Levels of Government and the Security of the RealmIt turns out that magic is involved -- the new railway lines are somehow affecting powerful and ancient ley lines which, in turn, may affect the entire country.  Clearly, James and Cecy have much more to contend with than a missing magician.

And Kate and Thomas have much more to deal with than twice the usual number of children:  Georgy has moved in with them, strangely stoic, and refuses to explain her sudden separation from her husband; there is a prowler who is able to cross the magical wards Thomas sets around the property; not only have the children been tormenting the servants by bringing snakes into the nursery, they also seem to have much more magical talent than had been previously thought.

As in the first two books, The Mislaid Magician is spun out in letter format over the course of a few months.  (If you aren't familiar with them, just imagine a cross between Jane Austen and Diana Wynne Jones.)

Like The Grand Tour, it isn't as strong as the first book, but it's a solid sequel.  It didn't keep me up at night, desperate to finish, but did I keep going back to it. 

Fans of the first two will enjoy, but I wouldn't give it to someone as a stand-alone.  For full enjoyment, I think it's particularly important to already be familiar with (and have affection for) the characters -- otherwise, things like Georgy being referred to as a 'watering pot' wouldn't be nearly as funny.