The book was a late replacement in the schedule, a result of Gary Russell's Instruments of Darkness being late in its delivery. The Shadow in the Glass was written so as to fit the title Instruments of Darkness, although in the end the title was retained for Russell's book, which was published later in the Past Doctor Adventures series.
Premise
Brigadier Lethbridge-Stewart discovers an odd mystery. A plane being shot down in World War 2 was nothing out of the ordinary...but the fact the crash site is still being guarded decades later is. The Doctor moves in to investigate this oddness.