That's always nice especially if it's not a much read book it's like OMG you've read it as well, I thought it was only me who had read this awesome book! 
This is where reading paper books comes to the fore, and people can actually see what you're reading - I can't be the only one who, when reading a great book in a public place, wonders if someone, anyone, would come up to me and say "that book is amazing, I'm so glad you're reading it"

(am I really the only one?

)
I've been known to say that to people at Waterstone's, if they're picking up a favourite book of mine, hehe - the crazy lady actually *talking* with people in shops
