1) The Harry Potter Series by JK Rowling. Yes, I know that technically, there are 7 books in the series, and therefore this answer is cheating, but you can’t really have one book without the others; they must be read as a unit, and can therefore be listed as one book.
2) Bram Stoker’s Dracula. My all-time favorite novel. I can’t tell you the number of times I have read this book, and have yet to grow tired of it. I still feel my pulse pound at certain places, and to be honest, there are parts of it that still scare me a bit, even though I know exactly what is going to happen the entire time.
3) Slaughterhouse 5 by Kurt Vonnegut. It’s an absolutely brilliant novel, and I think well-worth the read. I couldn’t read it over and over again, but it would be nice to read once a year or so.
4) Anything in the Redwall Series by Brian Jacques. For the same reason as #s 1 and 3. Brilliant stories, and beautifully told. IF I had to narrow it down to one book, I would choose The Long Patrol mostly because there are rabbits.
5) The Vampire Chronicles by Anne Rice. The first four books, as the rest aren’t worth it from what I’ve heard. But, I choose all four for the same reason as #1; while it is a series, it’s harder to read one without reading the others that preceded it.