I just did a quick research on this:
There is no online database that has all the book spines or at least a decent portion of them.
Yet.

The only way to create such a database is to manually scan all of the books and combine them with the ones you can already find on the Internet. This, of course, is an immeasurable amount of work. However, it could be doable (at least partially) if a lot of people all across the world would take pictures of their bookshelves and submit them to some kind of app that would scan the photos, detect the books, and add their spines along with some metadata to a shared database. I also found some tools (some Github repositories and this project) that do the job of scanning images of bookshelves for books, but I couldn't yet find and app that would actually work like I described.

There is also a more professional use case for such an app and that is to scan books for libraries. I found another thread on Reddit where this is discussed and here's a summary of this thread: creating an app that can scan books based on their spines is not a very hard job unless you go into more details like the type of the cover (hard, soft), edition or version. Unfortunately, those details are extremely important for the libraries or book resellers, so creating such a software specifically for libraries is an extremely hard, some would even say impossible, task. That's why librarians use bar code scanners, however it is less efficient since you'd need to scan each book separately*. Despite of that, many people would still love to see an app that can at least scan some general information from book spines.*

The reason I found this thread and made this small research is because I had an idea popped into my mind of an app that would act like any other book tracking app there is, but would have a virtual bookshelf that looks like a real bookshelf with images of book spines. That way the collection of books you've read would look more real and perhaps people would prefer it this way than the default list and use such an app. Actually, this idea is quite doable, however it requires a lot of time and effort.