Through Amazon, I bought a used copy of the book "Fear and Trembling" by Kierkegaard, in an edition that had been recommended to me by a friend (Translator of that edition: Walter Lowrie). When we met, her eyes fell on the name on the first page of my copy of the book: "Lois!" She exclaimed. "Where do you know her from?" - but I had no clue who that Lois person was. I said: "I don't know. It's just a name. Some previous owner of my book, obviously." As it turned out, she was my friend's fellow student in college many years ago. They were both in a course in which they studied Kierkegaard.
My friend was amazed at the coincidence. But how amazing was it really? What are the odds?
One might imagine that reading Kierkegaard is so rare that all Kierkegaard readers are at distance 2 from one another, in a tiny-world phenomenon. But that's not true. A few weeks ago a random colleague of mine came to my house, saw that book and started discussing it: that suggests an abundance of Kierkegaard readers.
Imagine that there are 6000 copies of that edition floating around the US. About 30 students took that course with my friend. The odds that I would end up with that book would still be 30/6000 = 1/200. That seems about right: low enough to be surprised by the coincidence, but not so low as to be in disbelief. Now the only information I need, to check my calculation, the the number of print copies of that book.
And that's where search engines failed me, to my surprise. How many copies of a book have been printed or have sold? That information is apparently not freely accessible. So I am left to my guesses about how amazed I should be.