that you’ve read. The number of people on earth who have also read those two books is surprisingly small. Add another book, and the number shrinks. Each book you add the number shrinks more than the last book, until at some point only you, out of all people on earth, have read that combination of books.
I wonder sometimes what is the smallest combination of books it would take to be unique.
The words written on your brain are written on yours alone…
Unless you only read Harry Potter, then you’ve got lots of company
Jonas David 
Lol. Literally the first thing I thought after your first two sentences was “Oh yeah, what if I pick Harry Potter?” But I was just being reflexively contrarian, and as usual, you were making an interesting point. I should think the number is one, if no one else has read it, but certainly no higher than two if you pick obscure titles in wildly divergent categories.