I noticed in one of Tom's recent posts that he was at the NYPL, going through the Carman papers there.
As Tom is no doubt discovering, that collection is very difficult to work with.
It's a VAST set (some 12 boxes, as I recall), much larger than the "Antietam Series" of Carman papers at the National Archives. The NYPL series is not simply Antietam, though. It's a lot of things: other Civil War manuscripts, personal papers, records from Carman's days in the Union Pension Office, etc.
And the "finding aid" -- such as it is -- is very imprecise. I found quite a lot of Civil War material in boxes that were listed on the finding aids as containing Carman's personal papers.
In other words, you have to hand search every box. (A massive, previously unpublished letter by Joe Hooker giving his thoughts on Chancellorsville, which I published some years ago in Civil War Times, came out of a box that the NYPL finding aid said contained only materials relating to Carman's family affairs. . . .)
And even when you FIND Antietam material, the library has added an additional roadblock.
Much of Carman's correspondence with Antietam veterans consisted of the soldiers returning to him a marked up section of battlefield map with a letter explaining those markings (a key to the map, essentially). But the NYPL separated the maps into one folder and the letters into another. So . . . you have unidentified maps without keys, and unidentified keys without maps. And because of library
policies about preserving the ordering of collections (ONCE some previous archivist has already made a mash-up of the original ordering), a researcher isn't allowed to simply put the two folders side by side and try to match them all up. (If in fact one could EVER make definitive matches of all of the paper.)
Which is part of the reason that you see the "Antietam Series" cited in the bibliographies of a lot of secondary works on Antietam, but little mention of the NYPL documents.
On the other hand, Tom will find a much better place to have dinner outside the NYPL than he will outside the National Archives, so it isn't all bad. :)