This is for EPUB.
Select Book in Book List, press 'U' - that will take you to the Unpack facility, it unpacks to a temporary folder, and opens it in your file manager, from there you can copy the html, css etc files.
An EPUB is a zip, so you could copy the EPUB somewhere, rename to .zip and open in with your everyday unzip utility.
Some 'MOBi's' can be unpacked but I've never needed or wanted to do that so I'm not sure about doing it. There's a
KindleUnpack plugin.
BR