The Terra Data (Dumarest of Terra #22)

That there was a planet called Earth only Earl Dumarest believed. He had been seeking it a long time ago, though nobody else at the crowded galactic center believed in it. He had acquired some bits of information during his search outwards: a Terrestrial zodiac, an authentic painting of Luna, a general series of hints that he was getting closer. Then he learned of a man who knew where Earth was, who had the special coordinates that would take a starship directly to it.

The man had lived on Elysius, and his window knew where his data were. She demanded a price from Dumarest - a chore that involved a mining expedition which would bring her back a fortune and him the information

But Elysius was not the Elysium its name projected - and The Terra Data not so rapidly come by!