I suggest that the best discussion on this subject is Appendix I to
_Landscape Turned Red_, by Stephen Sears. He discusses, at some length,
the question of whether or not Lee knew McClellan had found the order or
not. Rather than go into great detail on the contents of a widely
available book, I will summarize in brief: Sears contends that Lee did
not know that the order was even lost, but he did know that McClellan had
received some important new intelligence information. In a Sept. 16, 1862
note to President Davis, Lee said nothing about the enemy having details of
his plans, only that he (McClellan) was advancing "more rapidly than was
convenient." When he wrote his final report of the campaign in 8/63, he
did mention the Lost Order, but this was after McClellan's report had
been made public; moreover, Lee did not at this time say anything about
having learned of Mac's discovery.
Incidentally, DS Freeman changed his mind on this subject. In _RE Lee_,
he says that Lee did not know of the order being lost until months later;
in _Lee's Lts_ he says otherwise.
Jim Epperson
epperson@math.uah.edu