I reread a lot of books. They're like "comfort food" for my brain, like watching favorite movies over and over. At any point I might be in the middle of a bunch of them and randomly pick them up again to continue. However, if I'm reading a book for the first time I tend to focus on it until it's finished.
Too many!
Like Jennifer_C, I tend to reread books, so I'm always in the middle of something. Right now I'm rereading an old favorite, rereading a classic I'm teaching this quarter, and I'm about a third of the way into Pride and Prejudice and Zombies.
And then I'll pick up fast little graphic novels in between becuase I feel guilty about not having time to finish my books in a timely manner. (To me, "in a timely manner" means less than a week. Ideally, 2-4 days).
It's interesting that so many here reread books, because most people I know read a book once and then get rid of it. I like to reread books too, and consequently have a nice library of books.