|After completing Cyrus' Chapter 1||Atlasdam||9,000 Leaves, Alluring Ribbon|
While you could start this Side Story as soon as after completing Cyrus' first chapter, it's better to save it until you complete all of the Chapter 3s, as the locations you need to visit are Chapter 4 towns/areas. You can begin this by speaking with the Passionate Reader inside of the library in Atlasdam. She read about a city of gold, but needs some more information about it. There are two possible solutions, with the first one located in Marsalim. You're looking for the Scholarly Youth, who is located at the top of the area to your right inside of the town. Inquire/Scrutinize the youth for the City of Gold info, then bring that back to the Reader to finish.
For the other solution, you will have need of either Ophilia or Primrose, so put one of them in your active party and travel to Grandport. Exit the town, to West Grandport Coast, and venture to the western side of the area. You should spot a lone NPC standing by a ledge, a Best-Selling Author, who is the NPC you need. Guide/Allure him into following you, then return to the Passionate Reader in Atlasdam. Speak to her with the author in tow to finish the quest.