The fifth word if my novel, Alien Fiction is
Well, that was anti-climatic.
I looked up the 5th sentence of the 5th episode of Xenonomicon, and that was actually written by KR Williams, not me.
The 5th law of the Code of the West (from the novel Silent Saint)
Thou shalt remember that somethings are not for sale.
That one is pretty good, but again I did not originally write it. The Code of the West is a real thing as documented by the Cowboy Museum.
I must give up around the 4th instance of things because all of fifths are terrible.