The Hitchhiker's Guide to the Galaxy has, in what we laughingly call the past, had a great deal to say on the subject of parallel universes. Very little of this is, however, at all comprehensible to anyone below the level of advanced god and, since it is now well established that all know gods came into existence a good three-millionth of a second after the Universe began rather than, as they usually claimed, the previous week, they already have a great deal of explaining to do as it is, and are therefore not available for comment at this time...

 
I read this a few years ago, when it was first released in hardback, and I really enjoyed it. I didn't find it as funny as the radio series, but I doubt much could honestly match that! I've now bought the entire Hitchhiker's series, and can't wait to read them all.