this always seems to happen to me. How do you handle it?

I ride the city bus. While doing so I pop my book open and start to read it is relaxing and usually a great end to my day. However there is always that person that sits across from you, sees you reading and says, "Whats your book about?" I always mumble something about whatever pops into my head first to make it sound as uninteresting as possible in the vain hope that the stranger across from me will stop talking to me and leave me alone so I can read on the way home. It never seems to end the way I am hoping though. They always and I mean ALWAYS want to strike up a conversation. I am old enough, tired enough, and cranky enough that these days my stock comment when this happens is, "Sorry I don't mean to be rude, but I don't know you and I am reading because I really don't want to talk to you. I would rather read my book. Thanks." I don't even feel bad about saying this stuff to strangers any more. what do others do when they want to be left alone while reading in a public space?