See the notes in Hartwig. He tracks McClellan's movements.
He despatched 9th Corps around 1200 to South Mountain and from 1400-1430 he was with Joseph Cox inspecting the lead division marching out to SM and giving verbal instructions to the lead division. When he returned to HQ (which was only established around 1400) the copy of SO191 arrived shortly thereafter.
As to single volumes - tricky.
Hartwig's 2 volume work (second volume still in preparation) is likely to become the standard. In the absence of that Joseph Harsh's "
Taken at the Flood" is likely the best work, but is best read with the two "companion" volumes. The great shame is Harsh was writing the book of McClellan's strategy when he died...