You can have multiple instances of editor running. So, there's no need to 'close' current book before opening another one. But you can't create a fresh instance via the editor's File->Open, you must create them externally - e.g. by selecting a book in the library and clicking Edit book, or via the OS Open With, etc.
I have the
Restore state . . . again preference checked. The state of each book I'm editing seems to get restored if I edit it again. If I want to close an editor instance I use one of Windows' shortcuts - e.g. Alt+F4, Alt+space,C etc. The File->Quit menu option will close the current editor instance, it does have a shortcut but I have set it to None.
BR