The Dead in their Vaulted Arches- Alan Bradley

The absence of Flavia DeLuce's mother, Harriet, has always cast a shadow, since her presumed death in a mountaineering accident in Tibet. Now, astonishingly, a specially commissioned train is bringing Harriet back to Buckshaw. But rather than putting the past finally to rest, Harriet's return is set to trigger a further series of bizarre and deadly events, as a most curious group of individuals converge on Buckshaw to pay their respects…
As Flavia begins to unravel the shocking revelations of Harriet's past she discovers an extraordinary tale of espionage and betrayal that seems to be the key to her own destiny