First was Osprey Campaign #225: Messines 1917: The Zenith of Siege Warfare, which dealt with the use of underground diggings and mining in the trench warfare of WWI.
Second was Osprey Men-at-Arms #373: The Sarmatians 600 BC – AD 450, another group of wandering tribes on the borders of the Greeks and Romans. Not quite as interesting as I'd hoped for.