The pop up is similar to video, it prompts you to resue or restart.
Center button cycles thru album art.
If you are listening to a book and then navigate to another content and start it, then come back to the original book you will get the popup. It will remember your place in the Audiobook content. So to set a bookmark just ff/rew / to the content where you wish to place the mark, pause, then select other content.