well, not *necessarily*. You can do this from the touch screen, and it may even make more sense that way.
When you finish a book, use the screen to delete it from the 'Currently Reading' category. When you start a new one, use the screen to add it to "Currently reading".
Doing this via the screen may actually make more sense/be easier than using Calibre to do it.
But you still couldn't get the thing to display "currently reading". (Sigh) Then I could switch between, say, 4 or 5 books as I work through them. Alas.