@kc7zzv - when I originally wrote this plugin many aeons ago I did take a look at that. IIRC there wasn't much consistency at the time that I saw on when it was an ISBN versus when it was something else. I do recall spotting that "A" + ISBN coding for some books but think I also may have found some exceptions to that rule which made it all seem a bit dodgy.
Combine that with the fact that I would suggest many users of this plugin will also be downloading using other metadata plugins that do return an ISBN, and that whatever ISBN the plugin comes back with, it is going to get "mangled" by Kovid's ISBN pool Worldcat logic, it just didn't seem worth the effort.
However I can appreciate that if someone only had the Webscription plugin enabled then they would like to get an ISBN if it was available. I'll take another look and see if I can find the cases that put me off the idea last time...