The NARA branches have the information. Most larger libraries will have the book (index) and microfilm. Most Family History Centers can get it for you (LDS).
My library has the one for Texas since we are in Texas. - The information contained will vary by state- Texas microfilmed all the pages. It was originally meant to count the Union survivors and widows to account for the pensions - but some states counted ALL the Veterans so in the index it includes "all readable entries" - the Confederates usually have a line drawn through them (Texas has a single line through but you can still read all the information)- or otherwise discounted.