I just finished The Tin Princess by Philip Pullman.

This is the fourth book in the Sally Lockhart series, even though Sally only appears in a few cameos.

It features the return of a character from The Ruby in the Smoke, Adelaide, who was presumed dead.  She's reunited with Jim, and then becomes the princess, then queen of a small European country caught between Germany and Austria.

It's a worthy sequel, full of the same great characters and conflict as the other books.