What is the strangest book that you own or have seen? Strange means odd to you, since it is a relative term. significant, means personal, or in general.
Of one my favorites is "Oneirocritica: The Interpretation of Dreams" by Artemidorus Daldiani, of second century Greece. I consider it strange, because then people believed that dreams meant something. Now, a great deal don't. My second is probably both, "Mark Twain's Aquarium; The Samuel Clemens Angelfish Correspondence, 1905-1910" which documents his letters to about twelve girls ages 10-16 (what he called "schoolage") and "Enchantment: A Little Girl's Friendship with Mark Twain" which was renamed to "Mark Twain and Me" after the Disney movie of that name came out. I find those strange because he is a well respected author, and a documented pedophile (unpublished personal diary mentioned in the Aquarium he says that he "worships" little girls) (very much like Charles Dogdeson "Lewis Carrol"), which is so against recent culture.
As for significant, those are mostly religious books on Jewish outlook. I consider them significant, because I changed my outlook after reading them.