See the Creating Plugins link at the end of this ==>>
https://www.mobileread.com/forums/sho...d.php?t=118680
I don't know if you can
force the PI to appear in the book list context menu, as that would fly in the face of a basic calibre tenet - that the user is in control of what features appear in what toolbars and menus. Many users never use the context (right click) menus in calibre or anything else.
Have a look at what other PI's do - I've found the Extract ISBN a good place to start - a search for 'icon' in the code should put you on the right track
BR