I looked up the author on Wikipedia. It says he died in 1902. Perhaps it was his intention to write the second volume, but he didn't have time or the publisher at that time was not able or willing to publish a second volume especially if it was incomplete. This is pure speculation on my behalf.
Looks like Cist was too busy after the first volume came out to produce a second volume. He did a bio on Thomas, did magazine articles (among which are Cincinnati with the War Fever and “The Romance of Shiloh), he was editing the last three volumes of Reports of the Society of the Army of the Cumberland (which he served as editor for all but two of the 17 volumes that came out from 1868 to 1885), was apparently an early pioneer in civil war battlefield preservation and served on the Chickamauga Memorial Association in which capacity he managed to get authorization for the Chickamauga and Chattanooga National Military Park. He served as president of the Ohio Society. And after the war he established a legal practice in Cincinnati and probably had to attend to that as well.
I think looking at things the magazine articles may be Volume II, or as close as you'll get.