If you want to move from Calibre to Libiry, you can use this tool to rename all of Calibre’s opf files and cover files from
- metadata.opf → book.pdf.opf
- cover.jpg → book.pdf.jpg
- cover.png → book.pdf.png
After that, you are no longer bound to Calibre’s folder structure. As long as your book file and the same-named OPF and/or cover file live in the same folder, Libiry will use them. If you move a book in Libiry, the OPF and cover files will be moved as well.