I continue to read and enjoy Cornwell's Civil War series. I do however, have problems with these books in two areas: 1) the same themes recur to the point of monotony; and 2) Cornwell is not all that competent historically.
With respect to the latter, he makes several errors in _Battle Flag_. These seem not so much due to ignorance, carelessness, or convenience, as to a desire by Cornwell to re-characterize history to fit his own prejudices.
For example, he attributes to Jackson an active role in troop management during the battles at Cedar Mountain and Second Manassas. The truth is quite the opposite; Jackson remained largely passive during both of those battles. That was one of his few notable weak points during the entire campaign.
Cornwell's treatment here is odd, in that he supposedly based the historical portions of this novel largely on John Hennessy's fine history, _Return to Bull Run_, which details Jackson's passivity during those battles at some length, calling his performance "mediocre".
If a bit of gratuitous generosity on Jackson's behalf can be excused, the unnecessary swipe Cornwell takes at Longstreet in the Historical Notes section is not so easily forgiven. Cornwell makes the statement that "Lee's victory might have been more complete had Longstreet attacked [sooner]." Hennessy explicitly expresses the opposite opinion: Longstreet and Lee independently choose the exact same moment for the attack, and it was at that precise moment when the Union forces were at their most vulnerable.
Cornwell is, in essence, another Jackson/Lee groupie who thinks all faults belong to Longstreet -- and he's willing to re-write history in order to advance his prejuduces. Stuff like that is fine for arguing about over beers, but it's dishonest.
Corwell's books are fun to read though.