An excellent source for CW unit histories – overviews, really – is Dyer’s Compendium.
A Compendium of the War of the Rebellion
E 491 .D99 1959
Vol. 3 gives the fundamental unit info, date of organization, battles fought in, etc. In the absence of a published unit history, it can give the researcher a nice quick overview of that unit’s contribution to the cause.
I’m sure the original has been digitized, we have a reprint out on the floor.
There is also a HUGE microfiche collection (E470 .C58 1990) with unit histories, diaries, etc. Each fiche has a unique identifier that you’d need to access a particular item. Titles on this microfiche are individually cataloged in ANSWER and there are guides to the fiche collection on the shelves in the stacks. You can get the unique identifier from either.