Check
http://www.ida.liu.se/~tompe/mobiperl
The current version is 0.0.25.
This page contains some information and links to the manual page for mobi2mobi. There are Windows binaries available. To tar file contains the Perl files and you must have a system with Perl installed to use them and you have to install some additional Perl modules.
This post
https://www.mobileread.com/forums/sho...1&postcount=65
give an example of how to add cover image and thumbnail.