"book is being prepared for reading on the server": what files are created?

When seeing this message: "book is being prepared for reading on the server", what files are being created on the server?

I run calibre portable on one computer and make an offline copy of it to another computer that then runs calibre-server. When web users access calibre-server and get the message "book is being prepared for reading on the server", I guess some new files are being created and cached so that opening the same book will be quicker. What are where are these new files that are created so I can avoid overwriting/deleting them when I do my offline copies?