I usually read more than one book at a time. It wasn't always like that though. I think it got this way when I had to spend almost 2 hours commuting to office (one way) in heavy traffic, smog, and all things not conducive to understanding or appreciating Milan Kundera and the likes. Also, the size of the book and the size of my bag had something to do with adopting such reading habits. So if I'm not carrying lunch, then I'll probably read a bit of Dicken's Bleakhouse ( a whopping 1000 odd pages or so). On the other hand, if there's a tiffin eating into much of my bag space, I'll have a slim volume of the Hitchiker's guide to keep me company.
Rather fascinating how my reading preferences are now dictated by 'what fits' rather than 'what's good.' Sig(h)n of the times, I guess.