Yeah, I bought it at a mall bookstore (new edition, obviously) about 25 years ago. It's a nice book, realizing that it's written in 1866 (and I think probably reprinted items from Harper's magazine, although I'm not sure), so it has a point of view contemporary with the war.
Originally published in two volumes: This has been reprinted severaltimes. There is even a paperback edition. Illustrations are, indeed, from Harper's Weekly, with a new text by Harper's editors Henry M. Alden Alfred H. Guernsey. Reprints copiesare readily available quite inexpensively from Amazon, ABE Books, etc.
It is also available online at Hathi Trust and elsewhere.