Douglas S. Freeman's Lee's Lieutenants is a cornerstone work for any Civil War library. This three-volume set was initially published during the WWII years (1942-45) by Charles Scribners Sons and according to Harwell's In Tall Cotton, "stands in its own right as one of the great works of military history." Praise is generally universal, though Freeman's tone occasionally becomes that of the Southern apologist. Nevins described it as "the ablest descriptive and evaluative study of the leading generals in Lee's army; massively documented, movingly written, highly authoritative, and faintly smug."
First edition copies seem plentiful, but like any books of this vintage, condition can be problematic. Look for the Scribner "A" on the copyright page to denote first edition status. Choice copies in jacket will obviously command premium prices. The pictured set, available here, features a rare Freeman signature and is available for a cool $5000.
The set has also recently been given the Easton Press leatherbound, gilt-edge treatment.