Where are third party books stored in Books?

I purchase books from other online vendors that I upload in Books (non-DRM epub files). I like to keep a copy stored on my backup drives as well but every once in a while forget and the only copy I have is in Books. I am connected to iCloud to sync across my devices.

Where do I find the files I uploaded into Books? I can't seem to copy them out of books and dragging and dropping them to my desktop doesn't work (error if you try to open them). When I locate the Books folder on my Mac the only books showing are the ones from the bookstore, not my imported files.