Operation Barbarossa: the Complete Organisational and Statistical Analysis, and Military Simulation, Volume I

This is a very interesting introduction to the author's "Barbarossa" series, in which he seeks to set out comprehensive OOBs for the various combatants in Operation Barbarossa  and to come up with a quantitative approach to modeling combat during that operation.  This book consists of a rather detailed description of his approach, ranging from modeling from individual weapon systems attributes up to division-sized units; rather than pure mathematical formula, however (although there are plenty of those), the author intersperses his description with historical justifications and explanations.  Askey's approach is essentially a modified version of Dupuy's QJM; while I don't recall the specific changes from the QJM, the overall approach is changed little, while the specific attribute values have benefited from Askey's focus on the Barbarossa campaign rather than "modern war", as did Dupuy.

Readers should note that this is NOT a history of Barbarossa and anyone seeking a description of the campaign should certainly look elsewhere.  Rather, this is a reference work which explains Askey's approach to modeling the Barbarossa campaign in considerable detail.