still a bug need to be dealt with ----- when a store has a book, but unavailable, press "更改为随时可借", and the "从我的书库移除" automatically changed to “添加至我的书库”, which is the same content as pre-defined in the storyboard.
solved this bug --- Reason: UIButton is very special component, when you want to set the button's titleLabel's text, you'd better use "setTitile:forState" instead of plainly "XXbutton.titleLabel.text = ...", and now, you can write any pre-defined title for a button in storyboard without mistakingly display it while running the app.