When I release a book, the count (4 weeks and all time) increases, but the "recent book activity" never changes. The 3 most recent ones that are shown, have been there for weeks.
Try registering rather than releasing a book - then you should get some new covers :-) Although it says Recent Book Activity it actually shows the Books On Your Bookshelf Most Recently Registered, whether by you or a previous owner! Plus, it's probably best to register the long way round if you want to see some changes. I've seen comments that at present using pre-numbered labels doesn't alter the covers because they date to when they were printed not used but I never use them myself so don't know! Your stats suggest you've registered thousands of books but none in the past 4 weeks, possibly not since the site changeover i.e. the start of the book cover scheme