I never do though (OK I did once, but they were sitting right next to me and we were both reading "Wind Up Bird Chronicle" one of my favourite books of all time. Anyone who reads anything by Haruki Murakami rocks in my book).
I really enjoyed seeing this woman reading Walk the Lines (it's about walking the whole of the London Undeground, overground) as I had interviewed the book's author, Mark Mason, for the blog and we had a lovely conversation on the phone about tons of Tube stuff - some of which didn't even get into the book.
The temptation to say "I've interviewed him for blog, he's awesome" was overpowering, but she was so engrossed in the book, I really didn't want to disturb her.
Do you ever give a secret wink or smile inside when you see people on the Tube reading the same book as you?