A book’s “provenance” is the chronological history of its ownership and can also include a study of how certain individual titles passed from one owner to the next.
Obviously, for the vast, vast majority of books, who owned the book in the past is irrelevant and, in fact, any type of previous owner’s signature, rubber stamp, or bookplate is generally viewed as a fault with regards to the book’s condition.
On the other hand, such identifying marks can be a positive is if the book came from the library of a person of note, someone connected to the author, or someone closely related to the book’s subject. In the world of Civil War book collecting, such previous owner markings can add to a book’s luster if that previous owner was a Civil War veteran or politician.
For example, I recently acquired a nice copy of John W. Headley’s Confederate Operations in Canada and New York (Neale, 1906) from a very reputable book dealer. The front endpaper of the book contains the signature of J. Taylor Ellyson and also the rubber stamp of J. William Jones of 709 ½ Clay St. in Richmond. The dealer pointed out that Ellyson (1847-1919) served in the Confederacy’s “Richmond Howitzers” and was later a three-term mayor of Richmond. Jones (1836-1909) was a Confederate chaplain and the author of Christ in the Camp (1887) and The Life and Letters of Robert E. Lee (Neale, 1906). He has been described by one modern historian as "the single most important link between Southern religion and the Lost Cause." Such ownership history adds cachet to any book.
So all in all, if you have a 19th-century Civil War book with an earlier owner’s signature or two in the front, it might pay to do a bit of research to try and find out who that person was!