Requested and got a review copy in the mail today. Glancing through it, it appears that Suydam - like a good number of his regiment - was captured at New Market Gap on May 13 by John Imboden's troopers and was sent to Andersonville. There are a couple of letters from the months leading up to Sigel taking the field, and one from a week after his capture but they are sparse on details. I'll be interested to see what sort of context the letters are placed into, as the bibliography is only about a page and a half, and about half of that is CW-related. Surprisingly, Jack Davis' Battle of New Market is not listed, although Valley Thunder is and so is Turner's New Market Campaign and Jerry Holsworth's Blue & Gray article about NM.
This one has just moved to the front of my stack of review titles (which also includes, in no particular order: Fergus Bordewich's America's Great Debate, Frederick Hatch's Protecting President Lincoln, and Meredith Baker's Richmond Theater Fire).