The symbol || appears on the inside of both the front and back cover of House of Leaves.  Collage #1 in Appendix II C shows that this is the Ground-Air Emergency Code signal symbolizing a request for medical supplies.  A few more of these symbols appear throughout the book (perhaps as directional cues for the reader, or in reference to Johnny’s father’s career as a pilot), but it’s interesting that the book should begin and end with a request for supplies: a call for help.  Other portions of the book (primarily Zampanò’s bits about The Navidson Record) reference and duplicate the Morse Code distress signal, ••• ‒ ‒ ‒ •••, hinting that members of the explorations are in trouble.  Perhaps the || symbol appears at the beginning of the book as a warning that its characters, its authors, maybe even its readers are in distress, and again at the end of the book as reminder that reading the book did not help: someone still needs to be rescued, the riddle still needs to be solved. 

-Laura L.