[arspr]

Just open another instance of the Editor.

(I always open new books from main Calibre window, not from open book inside the Editor. In this way a new instance of the Editor is launched every single time. In fact you can have even the very same book open several times at once!!!

Of course, for this procedure you need the book you want to edit to be included in the Calibre Library. But adding books to the Library is in fact the very first step I do with any new book. I cannot think about having lone books in my system without being managed by Calibre. Even the true folder and name structure Calibre uses for storage is really wonderful and neat even for an outside-of-Calibre browsing session).