Willis actually went to the trouble of explaining time travel - to some degree - in that book. Many precautions are taken to make sure no one sees travelers arrive and no one's life is radically altered by the his/her stay (because changing the past is detrimental to our present well-being, duh).
[insert in your mind the picture of The Doomsday Book that blogger wouldn't let me upload]
Many, many years after that delightful book won Willis the Hugo and the Nebula (scifi's two biggest awards), she's got a sequel out: Blackout/All Clear, which was split into two books. I'm guessing it's because Blackout is 491 pages on its own. I bought this one over a year ago, when it was still in hardcover. It's very heavy.
I've heard some great and some not-so-great stuff about this one. Whereas her first book featured Kivrin's trip to midieval England, this book goes to World War II England. So far all I know is what the dust jacket says: a 17-year-old boy is in love with an older student, and he wants to go back in time to "catch up" with her in age, so she'll take him seriously.
Blackout/All Clear has been nominated for a Nebula, and considering that, may be nominated for a Hugo, too. How cool would it be to have a book and its sequel both be worthy of scifi's two greatest awards? Sequels are so rarely worthy. But I'm hoping this one is.