where are my epubs? they show up in books.app, but where is the actual file?

I have purchased books from other places and have made a few of my own from pdfs that I had. Someone at work wants to read one of them- and I cannot find it to send to them. it is in my books.app, but I cannot find the physical file, those seem to have been moved off the computer. I found the book artwork, but I cannot find the book itself.

latest version of software

I looked in ~/Library/Containers/com.apple.BKAgentService/Data/Documents/iBooks but all I can find is the audiobooks