Don't you hate that question? I hate when I'm reading on the tram or walking someplace and a stranger asks me what I'm reading. The book should pose an imaginary boundary, a wall that one should not try to penetrate. Especially a stranger. It's not rude to read a book in public, but it is rude to interrupt someone who is absorbed in their reading. Whether its the newspaper, book, magazine or bible*.
I've often taken my book on my regular walk to collect the kids from school. It's a route I know very well and can manage it with peripheral attentions quite easily. As soon as someone speaks to me - that's it you've overloaded my senses and put me at risk of stepping off the curb into traffic! Hope you're happy, you nosy prat! I exaggerate slightly. Rarely has someone aked me while I've been an active pedestrian. On trams, trains or buses however - all the time. Why they can't just do what a normal, polite person should do and crane their necks a bit to peek at the book's cover, I don't know. I could be mildly egotistical and say, perhaps I'm so stunning that they find the book question a wonderful way to enter conversation and perhaps more? Because "Whatcha readin'?" is a great way to open a conversation because the next question is "What's that about?"
"What's that about?" is the crux of the conversation. How you respond will determine if the conversation continues or stops dead. You could be honest or you could lie. You could stop the attempts cold and respond "You know, I just want to read my book, excuse me." but that will make for an uncomfortable trip until one of you exits the transport. Unless - please no - you get off at the same stop. At which point I'd have to immediately walk to the nearest shop in fear of being stalked. If you answer the question, the person will either inquire more about it if you sell the story well, or back off in conversation because you've failed to impress them. Aren't they obnoxious?! How dare they judge us for what we choose to read?! Who invited their opinion anyways? ARG - I hate them! And their book taste/ pompous judging ways.
Best thing to do, is read your book with headphones in place. Not on of course because if you happen to be walking you still need to be mindful of traffic and pickpockets. Also of when your stop is coming up on transport. But as a tool to discourage the "Whatcha readin'?" types from getting a chance to pass judgment or think they can get in your pants. Don't let them get a chance to think themselves smugly clever at our expense while interrupting our read.
*Which is a book and yet I offer it up in its own category because this becomes questioner's beware as bible readers are probably the only people who love to be asked "Whatcha readin'?" because then they get to tell you in all its glory and we shouldn't encourage such behaviour. Then again, serves the questioner right.