The Colour of Magic by Terry Pratchett

The Colour of Magic : Hardback : Colin Smythe Ltd : 9780861403240 : 086140324X : 10 Nov 1989 : On a world supported on the back of a giant turtle (sex unknown), a gleeful, explosive, wickedly eccentric expedition sets out. There's an avaricious but inept wizard, a naive tourist whose luggage moves on hundreds of dear little legs, dragons who only exist if you believe in them, and of course the edge of the planet.