Type this into search box:
Then type READ into the Saved Searches box to the right of the screen and click the green + symbol to save it. You now have a search. Go to the pull down on the far left of the screen and select READ and only books you have tagged as read will appear.
To make read books disappear create another search with the word not in front of the tags (not tags:read)and save that as UNREAD.
Hope that helps