All the Grey Cats (1988)
Aubrey returns after a three year gap, although in the timeline of the books it's been less than a year. The powers-that-be don't trust Aubrey after the events of The Bear's Tears and give him the unimportant job of overseeing the defection of an East German who just happens to be the son of an old enemy, Brigitte Winterbach.
We have not met her in any of the previous books but there is a backstory hinted at throughout the novel, with a young Aubrey encountering her and her scientist father at different stages before, during and after World War II. It gives me the impression that Craig Thomas has an incomplete novel in a drawer with just that story!
Meanwhile Timothy Gardner, son of the main protagonist of Wolfsbane, is also finding times hard. He was an officer of the Gurkas which has been disbanded (in the novels). He's hanging out in Nepal where he stumbles on a Russian/East German plot to invade the country.
It's not long before he's on the run, from his own side as much as the East Germans. (This is a theme that last appeared in The Bear's Tears and will resurface again in the next book.) He's trying to find proof about what the Russians are up to while the British don't believe it. So it's up to Aubrey to save the day.
British traitor Andrew Babbington is in exile in Moscow where he is overseeing the Russian side of the operation. KGB man Priabin ( Firefox, etc.) is now based at the Soviet's London embassy.
And just to finish the British edition of the book on a high note, Patrick Hyde is missing, feared dead, in Afghanistan. Hyde gets mentioned throughout the book yet the US edition (retitled "Wildcat") has all such mentions deleted. Very strange...
Second book in the 'Babbington trilogy'