The Light Fantastic
At the end of the first book they had just fallen off the edge of the world, but it seems that the discworld isn't going to let them go so easily. The world rests on the backs of four elephants standing on the back of a giant turtle, and it seems that the turtle is behaving rather oddly, and the spell lodged in Rincewind's brain is needed to put things right. So with a bit of magic the pair are back to dashing around the discworld, narrowly escaping death at every turn.
The first book was a collection of loosely related mini-stories. This one is more of a single narrative, but don't expect too much from the plot, otherwise your expectations will be upset with each random twist. You need to be a fan of the author's unique style of humour, such as Twoflower teaching Death and the Four Horsemen to play bridge, or, when Rincewind is looking for food in a wood he hears a seemingly disembodied voice telling him where to find some onions. 'You're not a tree are you?' he asks. 'Don't be silly, tree's can't talk: I'm a rock'. If that's you like that sort of humour then you'll find that Pratchett really gets into his stride in this book.