What's the difference with the original edition?
As explained in the preface, the 1st edition dated from 1995, this one dated from 2015. Between this 2 dates, lot of soviet archives has been opened, from STAVKA, GKO, NKVD... so the book became more accurate, about battles, statistics etc...
...The twenty-six tables in the book’s second edition (versus the five in the 1995 edition) attest to the magnitude of new statistical evidence describing the scale of Soviet participation in the war. Among other things, the new edition broadens the definition of available manpower on both sides, including such overlooked groups as NKVD troops, Axis satellite armies, and dissident forces that the Germans recruited in the non-Russian portions of the USSR. It rejects traditional efforts by German generals to hold Adolf Hitler solely responsible for all German errors and atrocities...
Glantz, David M.; House, Jonathan M.. When Titans Clashed: How the Red Army Stopped Hitler (Modern War Studies) . University Press of Kansas. Kindle Edition.