[kovidgoyal]

I dont see what difference it makes. With your suggestion, if the mouse slips you end up creating an unwanted duplicate in another library, which means you will have to switch to that library and delete it. With the original scheme, your book gets moved instead of copied to the correct library, an error you can remedy by simply switching to that library and copying back, later. In either case you need to switch to another library, find the book and perform an action on it.