Not all Editor panes are visible at one time. The Editor layout includes some panes that are "tabbed" for opening and closing. The layout can be customized, giving maximum access to the information you need.
The default location for its "tabbed panes" is on the left side of the Editor window. While you are working with a book or topic, you can change the layout by moving the nested tabs to another location, or by separating and displaying individual opened panes.
A tabbed pane can be:
Any modifications you make to the layout of the tabs in the Editor will be saved and used the next time you open an object of that type (book or topic). The layouts used by the Editor when opening a book and opening a topic are saved separately, so changing one will not affect the other. You can therefore have two Editor layouts - one for working with topic content and one for working with book content.
Use the Reset Panels button on the Editor's View ribbon to return the tabs to the default layout, or if a tab is accidentally "hidden" after being moved.