Coincidentally, the jacket background of the third in the Promise of Gold trilogy, Dearest Enemy, is one such image.
The original is panoramic, so this is just a fragment of it.
According to Wikimedia commons, where I first found it, it was used as an illustration called "Sacramento-City in California" in a book called The United States Illustrated, by Charles A. Dana (New York, 1855).
That is by no means the only book where it has appeared, and there are plenty of copies of the original lithograph, too. It is featured on many public domain sites, including the Library of Congress, which gives a detailed history of the engraving.
Title: Sacramento city, Ca. from the foot of J. Street, showing I., J., & K. Sts. with the Sierra Nevada in the distance / / C. Parsons ; drawn Dec. 20th 1849 by G.V. Cooper ; lith. of Wm. Endicott & Co., N. York.
Cooper, George Victor, 1810-1878 , artist
So, there is a rich repository of images that can be used for book jackets, many of them in high definition, and many ways they can be found.
The list, given by GalleyCat:
1. The Library of Congress’ image collection to find public domain images
2. American Memory Project to find public domain images
3. Archive.org for public domain books
4. Public Domain Images Google Search
5. Illustrated books on Project Gutenberg
6. Creative Commons search on Flickr