I'm a bit of a bookworm (as you may have noticed), and would love to be able to do this - do you know how many more books I could get through if I could use the time I spend walking around to read?!! Who cares about stupid scenery (Edinburgh Castle? Pah!) when I could be READING, people!
I read when I'm stationary. Bed is my favourite place for reading but failing that I'll read on the bus or train, at my desk during lunch-break, or in the dentist's waiting room. In short, anywhere I can get a seat and 5 minutes relative peace.
I was thinking about the mechanics of trying to read a book while walking along the street, and I don't think you could read anything more challenging than, say chick-lit, or a non-fiction book about deerstalker hats. You'd have to constantly check where you were going, what you were stepping on, who was coming towards you and whether any cars look like they might mount the pavement and squish you. I think I could just about manage The Compendium of British Caterpillars/The Comprehensive Guide to Shipwrecks of the North East Coast: Volume Two (Volume One was fascinating, but sequels are never as good) or something similar, although it would take all my powers of concentration to absorb anything I had read.
If I tried to read anything in the realm of Flaubert/Sartre/Dostoevsky I would end up getting lost, stepping in dog poo and probably getting hit by a bus after stepping into the road without looking. Not that I'm suggesting I read Sartre even whilst stationary. That would just be silly.