Once you Teleport to Onett, go to the library. Talk to the guy who is wearing the mask. He used the Overcoming Shyness book to overcome his shyness, and he'll tell you to search the bookshelves here to find the book.
The room with the book is on the first floor, closest to the entrance. Check the shelf on the left.
Once you're done getting the book, go visit home if you want to. Then Teleport back to the Tenda Village. Take the book to the chief and he'll read it to everybody.
Afterward, the chief will give you Tendakraut and ask you to confirm the name that you told Tony back in Toto. Then you'll get a bag of Dragonite. Talk to the guy near the Tenda chief to have another cup of coffee and read another monologue from the secret narrator.
Now that the Tenda aren't shy anymore, you can talk to the Tenda merchant. However, he doesn't accept money in exchange for things. Instead, he will trade with you. For each Horn of Life that you give him, he'll give you the next item on his list. (I had always thought that this was some kind of bug that didn't show the selection arrow in his list of goods. Oops.) You can teleport to Saturn Valley to stock up on seven Horns of Life to get the items from the merchant.
When you're ready, talk to the strong Tenda next to the boulder, and he'll toss it out of the way. Then climb down into Lumine Hall. A person-shaped rock will say that you should go talk to the most talkative rock, who lives in the Lost Underworld. You can get there by going through Lumine Hall, so keep going.
After getting into Lumine Hall, go up, then take the path downward. You'll find a ladder. Go down it to find another talking rock, and a Super Bomb. Go back up the ladder and head west. Ignore the hole, then go down the ladder at the far left. Go to the very bottom of this room for an IQ Capsule. From that gift box, go use the nearby ladder. All the way at the right is the Diadem of Kings.
Return to the ladder, go down, then go north and use the northmost ladder. From there, head right, then go north. The path will curve around. Go straight down when you can, and you'll find a Luck Capsule. Go down the ladder nearby. Talk to the stone if you wish, then go around the path. Get the Rock Candy along the way.
Go up the ladder to get the Cup of Lifenoodles, but ignore the hole and just return down below. Get the Bottle of DXWater, and go up the ladder north of that gift box. Go left, then down the ladder that you find, and go southwest until you reach the Luxury Jerky. Go back to the ladder and go up.
Head southwest, past the glowing Your Sanctuary boss, to get the Rabbit's Foot. Then you can fight Electro Specter. Find out how to defeat Electro Specter.
Walk past where Electro Specter was, and drop down the hole. Walk along the left until Ness stops to look. His thoughts will scroll along, illuminated by the bioluminescent stuff on the wall. You'll receive the next Sound Stone melody.
Continue left to find a hole, and drop down to reach the Lost Underworld.