While I'm definitely no fan of McClellan, after reading Carman's The Antietam Campaign some of what he did during that campaign now makes more sense to me. Carman was actually there, and isn't really a McClellan advocate, but makes a good case that at least some of the criticism he faced later was misplaced and politically motivated.
I found the sections concerning the speed of the AOTP's advance from Washington particularly interesting, along with the bit on Harper's Ferry. Well worth the read.
He was still a tool, though.