The Light Fantastic by Terry Pratchett

The second Discworld novel.
 
When the very fabric of time and space are about to be put through the wringer -- in this instance by the imminent arrival of a very large and determinedly oncoming meteorite -- circumstances require a very particular type of hero. Sadly what the situation does not need is a singularly inept wizard, still recovering from the trauma of falling off the edge of the world. Equally it does not need one well-meaning tourist and his luggage which has a mind of its own. Which is a shame because that's all there is.