In olden days when people used to read printed books, if you saw someone reading in public, you might be able to make some guesses about them based on what they were reading.
(Those guesses might have been wrong, of course.)
And in particular, you might be able to get a sense of whether they shared your tastes in fiction; and if they were reading something you were interested in, you might be able to strike up a conversation.
(Of course, some people hate that; they're busy reading, they don't want to be interrupted by a stranger who wants to talk.)
I always had this vague fantasy that if I read books that I loved in public places (like, say, on an airplane), some attractive person who shared my tastes would notice me and we could talk about books. The fact that this has never happened to me, in twenty-some years of reading in public, was no deterrent to the fantasy—to the point that when I'm reading something that I'm embarrassed about (like military sf) in a public place, I sometimes try to make the cover less obvious.
It occurred to me today, reading The Deed of Paksenarrion on my iPhone while waiting for my plane, that those days are over. In the new world of ebooks, you can't tell what someone is reading by glancing at the cover; the cover will only tell you what technology they're using.
Which, of course, can be a way to strike up a conversation as well. “I see you have a Kindle; how do you like it?” I suppose bonding over a shared liking of technology is at least as likely as bonding over a shared love of, say, Le Guin. (I've certainly had many more conversations with strangers about my gadgets than about my taste in fiction.) But I think a person liking Le Guin is probably a more reliable indicator of my liking them than liking gadgets is.
Besides, in the post-print future in which this blog entry is set, it won't be surprising any more to see someone reading from a Kindle (or an iPad, or whatever else), so that avenue of discussion will likely be blocked.
I suppose strangers might still bond over books in one circumstance, though:
“Wow, I see you're reading an actual paper book! Are you [lowers voice] a bibliophile? Me, too! Wanna come over and see my collection?”