After some misadventures, Varvara becomes part of a group of journalists and other hangers-on, following the Russian Army in the campaign running from the siege of Plevna to the Treaty of San Stefano. One of this group, it turns out, is a Turkish spy, but it takes several twists and turns before we find out exactly who.
With duels, chases, and other feats of derring-do, there's plenty of action in Turkish Gambit and its plot is fast-moving and engaging. It has a good assortment of engaging characters in addition to our two leads. And it is set against a back-drop of battles, diplomacy and treaties which adds some interest, especially since the historical setting is not well-known in the English-speaking world.
There are plenty more Erast Fandorin novels — Akunin is apparently trying to write one in every mystery sub-genre — and I'll certainly be grabbing the others if I spot them secondhand or on sale.