My library pane is missing, how to get it back

XCode 11

Click the button highlight in the image.

Xcode 11 and 12: Click the "plus" button at the top, left of the "Show/Hide Code Review" button. Alternatively, use "View > Show Library" or hit Shift-Cmd-L.

Xcode 10: Click the "square in a circle" button at the top, left of the "show standard editor" button. Alternatively, use "View > Libraries > Show Library" or hit Shift-Cmd-L.