One book at a time? Or more than one? If more, are they different types/genres? Or similar?
(We’re talking recreational reading, here—books for work or school don’t really count since they’re not optional.)
Mainly I'm a monogamous. But occasionally I do cheat and read other books, leaving the other behind for a while. I don't like it, and I wouldn't do it as a general rule. I used to do it a lot and I would end up leaving some of the books unfinished, cause I was too engrossed by the others. If my attention is grabbed by one I might lose interest in the other, even forget about it. Also I think the best way to enjoy a book is to be totally absorbed by them. Something that can only be done once at a time.
That said, it happens sometime, of course. When you're a bookseller and see new books every day, shiny and tempting, it's hard to say "you stay there and wait for your turn". So what I do is try and be quick, so I can move on to the next. If I really can't wait, then I read it in the shop, without buying it, or even bringing it home. So I know that my current reading is still in charge and that one is only a distraction, a whim, that won't last long...