A lengthy aside: I remembered them this weekend after encountering Gary Gallagher's collection called the Antietam Campaign - a naively titled book, given the happenstance nature of the Antietam battle.
Joseph Harsh at length and Ethan Rafuse briefly did well to put paid to the idea that Lee had any initiative in the Maryland campaign - not just after the discovery of SO 191 but from the time Union forces decamped from the fortifications. Once Pope refused the field command, it became McClellan's campaign and devolved on his intentions. Lee's choice of defensive ground defined the battle but not the campaign.
If no one has done better with McClellan's activity up to the point of the discovery of the SO than Harsh and Rafuse, no one has done better after the discovery than Reese in that he produces the key insight about McClellan's campaign-level operational intent. He opens the door to what was to be.
Generations of incomplete analysis of McClellan's orders to Franklin have left us at the mercy of Gallagher-like Antietam Campaigners, compilers and summarizers with a painfully truncated view of Union commander's intentions - writers who can't see past the climactic final battle.
Reese's books are thus worth the deepest consideration.