The Last Continent by Terry Pratchett
Published by Corgi

At the Unseen University the wizards are trying to find a cure for the Librarian, who changes shape every time he sneezes.   Meanwhile, on  continent EcksEcksEcksEcks, Rincewind is doing his best to avoid danger, when he is approached by a kangaroo, who tells him he is destined to save the continent.   He reacts instantly - he runs away.   Back at the UU wizards climb through a study window onto the beach of a small tropical island several thousand years in the past, where the trees grow cheese and biscuits, chocolate coconuts, cigarettes etc.   They find a tree whose fruit is a boat, complete with sail, and set off towards continent EcksEcksEcksEcks.   Will they meet up with Rincewind?   Will he save the continent?   No worries!