It's not everyday you will see a mockup on Covervault showing only the spine on books. However the request for this style has been on the rise.
5 x 8 Paperback Book Mockup for your next book series. Read & Download
How about something a little different. Let me know how you like these multi-book presentations. Please leave me a comment with your ideas. Read & Download
Ya baby! Here's a 5 x 8 Multi book set mockup. Easily customize how many books you want by turning off/on the layer groups. This one is pretty straight forward. Read & Download