Dumarest, a native of Earth, describes it as inhospitable and scarred by ancient wars. He left as a child, having stowed away on one of the infrequent trading vessels that came to call, and has long since forgotten the return route and the coordinates (if he ever knew them at all). To nearly everyone he encounters, the name “Earth” is only a synonym for dirt, except for a few who recognize it as the name of a fictional legendary paradise like El Dorado or Avalon.

It is implied throughout the series that the Cyclan has something to do with all knowledge of Earth having been lost or destroyed in the distant past, and that they still strive to keep that knowledge secret. This hidden agenda puts them at odds with Dumarest from the outset of the series. Not until Book 32, The Return, do we learn that the Universal Brotherhood doesn't want anyone to find Earth either, and may also have had a hand in erasing records of that world from navigational almanacs. At least two other organizations, The Original People and The Terridae share Dumarest's goal of finding and returning to Earth, along with occasional treasure seekers and archaeologists.

Here's a chronological list of the clues to Earth's location Dumarest finds in the novels.

1. The Winds of Gath. Dumarest hypothesizes that Cyber Dyne's attempt to kill him has something to do with his knowledge of or quest for Earth. 
2. Derai. Derai tells Dumarest that while in a Cyclan institution, she learned that the Cyclan know about Earth (confirming his suspicions from Book 1, The Winds of Gath).
3. Toyman. Although the most important details about Earth are destroyed when the famous computer on the planet Toy is sabotaged by the Cyclan, one fragment is salvaged – Earth is also sometimes called “Terra”.
4. Kalin. None.
5. The Jester at Scar. Jocelyn confirms again that Terra is another name for Earth.  He also mentions an ancient poem that suggests that all humanity originated on one planet - Terra.  He surmises that if true, then the first wave of colonists to expand from that planet would have used it as the zero point of their coordinates (instead of the center of the galaxy, as is currently used).  If Dumarest can find an ancient star almanac or ship’s log that uses the old system, and correlate at least three planets from that source with planets whose coordinates are known in the current system, then the coordinates of Terra can be extrapolated from that information.  I personally speculate that the poem to which Jocelyn refers may be a fragment or variant of what Dumarest eventually learns is the creed of The Original People, but since Jocelyn doesn’t recite it, it also might be totally unrelated. 
6. Lallia. Nimino tells Earl about The Original People just before he dies. More importantly, the shrine gives him the sector of the galaxy in which Earth lies.
7. Technos Elaine Delmayer remembers an ancient nursery rhyme from one of her father’s old texts, which includes a mnemonic for the signs of the Zodiac.
8. Veruchia. The log from an ancient ship contains no reference to Earth, but seems to use a different galactic coordinate system than the one in current use.  He already knows from Jocelyn, Lord of Jest (from Book 5, The Jester at Scar) that if the old coordinate system used Earth as its center of reference, a computer could extrapolate the location of Earth by comparing the old and new coordinates of three worlds for which both sets are available.  However, this doesn’t seem to pan out in the following books, so either the log used yet a third reference system, or there were less than three planets listed that Earl could match with their contemporary names, or it was stolen, lost or destroyed before he could do the cross-reference.
9. Mayenne. Nothing new, although legends of the existence of Terra and The Original People are confirmed by a scholar traveling on the same ship (who later turns out to be Mayenne’s partner, an agent of the Cyclan, who was using this knowledge to distract Dumarest’s attention until he could be delivered to the Cyclan).
10. Jondelle. Jondelle’s biological family and/or those sponsoring the genetic research may or may not be members of The Original People.  As a reward for rescuing the boy, Jondelle’s grandfather cryptically tells Earl his investigation of old legends reveal that Earth must lie in the seventh decan, orbiting a yellow, G-type star.
11. Zenya. Salek tells Earl the names of certain stars rumored to have been in the same vicinity as Terra: Sirius, Polaris, Alpha Centaurus and Procyon, and the fact that Polaris is on the imaginary line formed by extending Terra’s axis.  He also tells him that he believes the Cyclan knows Earth’s exact location.
12. Eloise, None, although Dumarest regrets escaping Instone in such haste that he failed to query Camolsaer’s extensive data banks about it.
13. Eye of the Zodiac. Dumarest meets a tribe who claim to be part of The Original People, and learns some of their beliefs and customs, as well as the fact that the sect is apparently fragmented (this particular branch has purposely forsaken technology and scoffs at branches that haven’t).  Dumarest views a holographic montage of Earth’s glory in the ancient past, prior to whatever calamity caused it to become the scarred, barren world on which Dumarest was born.  Most importantly, he learns the distances of Earth’s closest stellar neighbors, several of whose names he learned in Book 11, Zenya.  Now if he finds any one of those worlds, it will narrow down the search for Earth considerably.
14. Jack of Swords. None.
15. Spectrum of a Forgotten Sun. Dumarest learns of an ancient sect called the Guardians of the Sun which may have originated on Terra.  He learns that the planet Emijar was probably settled by colonists from Terra (and based on which I personally speculate the Guardians of the Sun may be related to The Original People).  Most importantly, he obtains a spectrogram of Terra’s sun.
16. Haven of Darkness. None, but Dumarest learns how expensive (and dangerous) it will be to do a brute-force computer search for Earth based on the clues he has accumulated so far.
17. Prison of Night. None.
18. Incident on Ath. The AI Hury is destroyed before Dumarest can get much useful out of it, but he does learn an astronomical symbol used to denote Earth, the fact that Earth is the third planet from its sun, the Earth’s exact circumference and diameter, and the composition of its atmosphere.  Exactly the kind of useless but detailed trivia that give the infuriating impression that Hury almost certainly did have useful data before it was destroyed.
19. The Quillian Sector. None.
20. Web of Sand. Jwani gives independent confirmation of some minor information Dumarest has already acquired.  Dumarest believes he may know more, perhaps even being associated with The Original People, but he doesn’t press for answers because A) The Original People don’t talk about Earth with outsiders, and B) Jwani saved his life.
21. Iduna's Universe. None.
22. The Terra Data. The late Rudi Boulaye also searched for Earth and apparently discovered the coordinates, but died before he could finance an expedition. Unfortunately, the medallion on which Dumarest believes the coordinates are recorded is destroyed before Dumarest can recover it. Dumarest learns nothing new about Earth, but Rudi’s research notes both confirm previously acquired clues and suggest that it may be possible to determine Earth’s coordinates without resorting to an expensive computer analysis of Sol’s spectrum (a search which is also highly dangerous since the Cyclan knows that Dumarest has tried to find Earth by that method before).
23. World of Promise. Dumarest attempts to retrace Rudi Boulaye's footsteps to replicate his research, but the trail has gone cold. Dumarest does learn that Erce is another name for Earth, as well as a few galactic landmarks to which it is near.
24. Nectar of Heaven. None, but Dumarest believes the original settlers from whom the current colonists purchased the planet Sacaweena were of the Original People (who called it Erce, perhaps out of nostalgia for Earth).
25. The Terridae. Dumarest discovers the Terridae, a fanatical sect that seeks Earth and awaits “The Event”, the discovery of Earth’s location, on their artifical planetoid, Zabul. The Terridae are aware of the Original People, but do not appear to be directly related to them.
26. The Coming Event. Dumarest's lover Ysanne deciphers clues Dumarest gained from the archives of the Terridae on Zabul, and deduces the coordinates of Earth. The coordinates of Earth!
27. Earth Is Heaven. None.  Worse, the coordinates Ysanne and Dumarest got from the Terridae's archives were not actually Earth’s, but those of the planet on which the Terridae was based prior to building Zabul. Dumarest does learn of the Ypsheim, a sect which fled Earth around the time of the cataclysm that turned it from a paradise into the barren, scarred world Earl knows.  Unfortunately, their legends contain no clues as to Earth’s location because their belief is the polar opposite from the Original People; the Ypsheim believe Earth is a living hell (actually worse than Dumarest remembers it being, and his memories aren’t very happy) and they have no intention of returning.
28. Melome. Dumarest recovers the memory of reading the captain’s journal from the ship on which he originally stowed away and left Earth. The journal mentions the world Ascanio, presumably close to Earth, as well as the fact that Earth is proscribed.  Also, Shakira gives Dumarest the name and homeworld of a man he believes can help in the search (the name turns out to be Chenault, but that isn’t revealed in this novel).
29. Angado. An antiquities dealer tells Dumarest where he can find Chenault and confirms some previously learned clues about The Original People (and may be affiliated with them himself).
30. Symbol of Terra. Dumarest finds Chenault and learns the name of the merchant company which owned the ship on which Dumarest stowed away from Earth, but the coordinates elude him yet again.
31. The Temple of Truth. Dumarest finally learns the (apparently genuine) coordinates of Earth!
32. The Return. Dumarest discovers the Universal Brotherhood does not want him to find Earth.  Although this casts some doubt on whether the Cyclan is solely responsible for the conspiracy to keep Earth’s location secret, Dumarest doesn’t care.  More importantly, he ends the book in orbit around Earth. In orbit around Earth!
33. Child of Earth. Dumarest has found earth, and discovers why it is proscribed.