Poul Anderson 
Longest Voyage [EPUB ebook] 

In search of the Aureate Cities, Captain Rovic had brought the Golden Leaper halfway round the World. Weathering hurricanes and mutiny, he meant to do what no other ship’s master had done: circulate the globe and return to riches and glory. Then, on a distant barbarous island, Rovic met a shipwrecked traveler who claimed to have come on an even longer voyage. But who could believe his tale – of a ship that sailed between the stars?