Mine are:
Desert Solitaire: A Season in the Wilderness - Edward Abbey
https://en.wikipedia.org/wiki/Desert_Solitaire
https://www.goodreads.com/book/show/...sert_Solitaire
{Non-fiction, set in Arches National Park, Utah; the lower half of letter "X" forms a triangular arch.}
Treasure Island - Robert Louis Stevenson
https://en.wikipedia.org/wiki/Treasure_Island
https://en.wikipedia.org/wiki/Treasure_Island
{Couldn't resist wanting to read it again; X on original editions' maps, and most every other treasure map since.}
Hard Times - Charles Dickens
https://en.wikipedia.org/wiki/Hard_Times_(novel)
https://www.goodreads.com/book/show/5344.Hard_Times
{I like Dickens and haven't reread any for many years; "times" as in multiplication as in "x" times.}