Shopping vintage isn't just a smart move for the planet; it can also be great design inspiration. Retro book covers are particularly interesting, with their unusual color palettes, patterns, and design. The goal of the Bookscans Project is to provide a visual catalog of all (yes, all) vintage American paperbacks but especially those printed before 1960 and those having a 25¢ or 35¢ cover price. And all the images are public domain (free) so they can be used in whatever way your inspiration dictates—artwork, art projects, paint color palettes... you name it!
The images are arranged by book number within their respective publisher folders. (The website is way dated, and the navigation is confusing, in my opionion.) Of particular interest: the Reference folder, which contains information for avid vintage book collectors (price guides, histories of paperback publishers, bibliographies, and identifying first editions, for example), and the Interpretations folder, which showcases different cover art for the same book.
Check it out! The Bookscans Database