Unable to find my Books EPUB files on iPhone.

I have a number of books on my iPhone (iphone 11, iOS 17). I cannot find the files. I see lots of folks seem to have this problem. My issue is I do not have a Mac computer. All I have is my iPhone and my iCloud account. The files are in neither of these locations. I do know the file is somewhere because Books regulalrly deletes the books (even though I have plenty of cloud storage), and I have to redownload them. Most of the books have not been bought through the Apple bookstore, so I cannot check there. Any suggestions Apple community?

[Re-Titled by Moderator]