For me, this question would be a nightmare as I love so many but the five I know I could not do without are:-
- The Lord of the Rings
- The Chronicles of Narnia
- The Collected Works of P.G. Wodehouse
- The Discworld Novels by Terry Pratchett
- The Collected Works of Agatha Christie
I expect I'd be accused of cheating by going for "collected works" but why not? Just put them together in one big volume and it would be one book, would it not?! Mind some weight training would probably be useful in preparation for trying to pick the things up to read them!