Here are the promo blurbs on the back covers of the official novels (might count as including a few spoilers).
The Stairway to Eternity 1 (2000/12)
"Reid, who takes a laidback approach towards life in submission to fate, and Farah, impetuous with a strong sense of justice—these two main characters launch a grand adventure story into motion on the lushly green and peaceful world of Inferia. As they gaze up at Celestia, another world that faces their own from the sky above, they see a glint of light hurtling toward them at an immense speed. But what exactly is that light? ...The first volume of a long-awaited new Tales novel series, brought to you by Sara Yajima! (A Namco Official Story)"
The Stairway to Eternity 2 (2001/01)
"A glowing ball of light bursts out of the sky directly towards the main characters Reid and Farah, who have been living peacefully in the Kingdom of Inferia. After a narrow escape, they find that a flying craft from Celestia has crash-landed, and a young girl named Meredy has emerged from the wreckage. She desperately tries to tell them something, but they don't understand her words. They set out on a journey together to visit their childhood friend Keele, a student at Mintche University, in hopes that he will be able to decipher her language. Eventually, they come to realize that Meredy's cry for help concerns a grave danger to their world."
The Stairway to Eternity 3 (2001/02)
"A girl named Meredy arrived from the neighboring planet of Celestia bearing the shocking warning that someone was trying to destroy Eternia. Hoping to avert this outcome, the three main characters Reid, Farah, and Keele cross over to Celestia for the first time, but they are soon confronted with the fact that scientific progress on that world has reached a far more advanced level than on their homeworld of Inferia. However, the key to preventing the disaster may not lie in the power of science, but in the very hands of Reid and his friends... This volume concludes the main story."
The Globe in the Azure Sky 1 (2001/04)
"Inferia is connected to Celestia via the "Bridge of Light." The King of Inferia commands Zosimos, director of the Royal Observatory of Astronomy, to search for that legendary bridge for the purpose of invading Celestia. Zosimos appoints Keele to assist in the search, having previously taken notice of his talent. After parting with Reid, Farah, and Meredy, Keele stays behind at the Observatory. His innate thirst for knowledge compels him to spend his days engrossed in research, but at the same time, he begins to feel faint doubts about the Kingdom."
The Globe in the Azure Sky 2 (2001/07)
"Keele immerses himself in observation of the Grand Fall at the behest of Zosimos, director of the Observatory. However, seeds of doubt gradually take root in Keele's mind the more that he witnesses the realities of the Kingdom, which include the director's cavalier attitude towards his research, and the selfish views of the royals in the palace. Swallowing his misgivings, Keele throws himself into his research, but one day, someone makes an attempt on his life... Who is behind this attack? And what is their aim?"
Dawn's Promise 1 (2002/01)
"After the Grand Fall, Reid and Farah head for Celestia in the Van Eltia to search for Keele and Meredy. But in the meantime, a series of disasters, natural and otherwise, have been taking place on Celestia... Will Reid be able to safely reunite with his friends? And what new crisis will they have to face? Sara Yajima's launches a long-awaited 2nd Eternia side-story series that takes place after the end of the game!"
Dawn's Promise 2 (2002/03)
"The Van Eltia, with Chat, Ayla, and Neah on board, set off for Inferia in search of aid, but it crashes into the ocean as a result of Neah's erratic behavior. Meanwhile, Reid has successfully reunited with his friends on Celestia, and is about to cross back to Inferia... But just what is about to happen on Celestia? The hit 2nd Eternia side-story series, depicting the post-game world, finally reaches its conclusion!"
Reid's Lizard (2002/09)
"* A story about Reid, Farah, and Keele when they were young. The three of them go to collect medicinal herbs for Farah's father who has come down with a high fever, but what happens on their trip?
* Moody, the playwright who appeared in the Eternia side-story novel The Globe in the Azure Sky, becomes acquainted with a young girl named Lucille, but then the Grand Fall begins!
* The story of when Meredy and Quickie first met. One day, Meredy sets her eyes on a fluffy little creature...
Enjoy these three stories in one volume as this 1st collection of Eternia short stories makes its debut!"
Keele's Dream (2002/10)
"* Meredy goes to Inferia to visit Keele, but that's when Keele is summoned to the palace!
* As Youdine prepares to investigate the "Bridge of Light" with Keele, he is taken back to his memories of when he first met Ras.
* Lucille stops by Inferia to return Moody's choker, and ends up meeting Joshua, whom Moody had told her about...
Enjoy these three stories in this 2nd Eternia story collection!"
Side Story: St. Elmo's Light (2001/03)
"On the world of Celestia, a hardscrabble land ruled over by thunder and darkness, the power of advanced Craymel technology reigns supreme. Aifread, the leader of a band of young thieves, takes his friends and heads for an abandoned mine in Luishka to dig up the ruins of the ancient Melnics civilization to sell for a lucrative profit. But to their surprise, they find that the Tinnsia Army has already staked out a position there, and soon enough Aifread and his gang are taken prisoner. And then something incredible happens to them!
The varied members of the gang of thieves include the apprentice engineer Maxton, the mysterious girl Rosania, and the barkeeper Morgan. A great adventure story unfolds featuring the man who would later become the legendary pirate Aifread along with his colorful band of friends. A novel based on the acclaimed game is finally here!"
Grand Battle on the Southern Seas! (2001/04)
"The two worlds, Inferia and Celestia, are predicted to be destroyed in the near future as a result of the "Grand Fall." Four people—the childhood friends Reid, Farah, Keele, and the Celestian Meredy—have stood up to save their worlds from this crisis. One day on their journey, a dragon-riding woman warrior named Maronne suddenly attacks the astonished foursome with a Sonic Blade and carries Reid away! A bounty hunter by trade, she seems to have taken a shine to Reid's sword skills for some reason...? The popular characters from Eternia go wild in the maritime city of Belcagne! On Belca, island of eternal summer, hot and spicy incidents just keep coming!"
The Gospel of Nereid (2001/05)
"Reid, Farah, Keele, and Meredy are stranded in the southern maritime city of Belcagne after a destructive hurricane damages boats all over the island. The party constructs a new boat designed by Keele, who has named it the Keele Zeibel, but as soon as they push it out to sea for a test voyage, the boat unceremoniously sinks! Wanting to warm up their chilled bodies, Farah and the rest of the girls head to a hot spring, but then... The novelization of the anime series based on the hit game comes to a dramatic end! The truth about the island gradually comes into focus, but what could it be?!"