E.C. Tubb 
Terra Data 
The Dumarest Saga Book 22

Only Earl Dumarest himself believed that there was ever a planet called Earth. He had been seeking it a long time — but nobody else at the crowded galactic centre believed in it. During his quest he had acquired some bits of information — a Terrestrial zodiac, and authentic painting of Luna, a general series of hints that he was getting closer.The he learned of a man on Elysius who knew where Earth was, and who had the special coordinates that would take a starship directly to it. Though the man was dead, his widow knew where the data lay. But she demanded a price from Dumarest — a chore that involved a mining expedition which would bring her back a fortune . . . and for him the information he sought.But the Terra Data would not be so easily come by!(First published 1980)