Percy Jackson and the Singer of Apollo is a short story written by Rick Riordan for Guys Read: Other Worlds. The short story was first released on September 16th, 2013. This short can also be found in the paperback version of The Hidden Oracle. It is also going to get a rerelease for World Book Day in 2019.
Using his tell-tale humor, Percy Jackson narrates the short story that tells us about the time when he was celebrating Grover’s birthday and Apollo turned up and asked him for a “little” favor.
That little favor turned into a big problem when Apollo sent Percy out to fetch his fourth celedon, who went rogue and ran off to the big city to try her hand at fame, fortune, and a solo career. 
Percy Jackson celebrates Grover Underwood's birthday party alongside Juniper and other dryads at Prospect Park in Brooklyn. After gifting Grover aluminum cans, the satyr suggests playing Pin the Tail on the Human, frightening the only human present - Percy. However, before the game could begin, Apollo arrives and greets Percy and Grover. They attempt to tell him that they're taking a day off to celebrate Grover's birthday, but Apollo interprets this as a day off to help him.
Apollo takes Percy and Grover to the side in order to introduce them to the chryseae celedones, a trio of golden automatons who sing in beautiful harmony and will perform with the god in a concert at Mount Olympus. Grover protests that they sound fine, but Apollo reveals that the celdeones are a quartet and one had gone rogue after Hephaestus' two-thousand year warranty expired. Percy complains that Apollo could retrieve the celedone himself, but he explains that he needs to perform a sound check for the concert and that heroes are supposed to run the gods' errands. He suggests for the two to search at the Theater District of Manhattan as celedones are always looking to be "discovered". Grover realizes that a celdeon singing in public could create mass panic causing Percy to agree to the quest, but asks why he picked the two to complete the quest. Apollo reveals he likes Percy and that his experience with the Sirens during the Sea of Monsters is a similar challenge. He summons his personal lyre and tells Grover that his immunity to magical music and ability to play the lyre's magic will aid in the capture the celedon. He sets the rendezvous at the Empire State Building by sunset before vanishing.
Percy and Grover (who doesn't bother to disguise himself) take the subway to Times Square as it is located in the middle of the Theater District and filled with tourists and broadway performers. While searching, Grover bumps into a hot dog vendor cart and clutches the lyre protectively. Percy questions the use of the lyre, but Grover explains that if the right song was played, it could create anything. Percy expresses doubt as Grover's skills on the reed pipes wavers between days. The satyr spots the celedon approaching a mic on an outdoor stage and it prepares to sing causing Percy to stuff wax in his ears. He explains the difficulty in fighting automatons with mortals and manipulates the Mist to show a presidential motorcade to the cops in order to block off the area.
As the police block off the exit, Percy uncaps Riptide and attacks. However, the celedon's song resonates through the earwax and causes mortals to weep in depression. Grover frantically imagines a cage around the celedon while playing the lyre, but instead summons a brick wall between Percy and the celedon. The momentum causes Percy to crash into the wall and topple it over the celedon who regains her compsure and begins singing a song about Apollo and the sun. Percy begins wrestling with the automaton but notices that the heat resonating from the song has melted his earwax and singed his shirt. Grover attempts to think of a cage for the celedon, but random shifts occur due to him being nervous. With no other choice, Percy swings Riptide forcing the celedon to stop singing and transform into a quail who flies to the top of Times Tower.
Percy and Grover ride the elevator to the top of the tower and climb stairs to reach the roof. They spot the celedon serenading Times Square with "New York, New York". Percy thinks of shoving her off the roof but reminds himself that she would transform into a bird causing an idea to form in his head and asks Grover if he can use the lyre to summon a birdcage. Grover protests that birds should be free, but realizes Percy's plan and promises to try. Capping Riptide, Percy asks Grover for the blindfold he was going to use on Pin the Tail on the Human and approaches the celedon. As she finishes singing her song, Percy jumps on her back and gags the celedon as Grover strums the lyre for a birdcage.
The celedon, however, attempts to throw Percy off the roof of Times Tower while Grover nervously tries to think of lyrics. A birdcage of Celestial Bronze begins to form, but the celedon spins Percy off her and pushes him off the tower. A metal rung catches his pants belt, but the momentum causes Percy to slip out of his pants and continue falling. The demigod snares onto a billboard to prevent himself from falling and reveals that he wears plain blue boxers claiming they're more comfortable than briefs. The celedon smiles down at Percy and begins singing about Percy letting go of the billboard and falling.
Percy wills himself not to listen and thinks of Annabeth and his saving of her from the Sirens. He imagines her anger and resolves that it is scarier than monsters, breaking the automaton's spell. Percy realizes that the celedon would only transform into a bird if she was startled and uses the empathy link to tell Grover his plan. He asks the automaton for an autograph and Grover tells her to check his pants for a pen. The celedon complies and uncaps it only to be startled by the appearance of Riptide. She transforms into a bird, but Grover grabs her and shoves her inside the birdcage. He laments that he scratched Apollo's lyre and imprisoned a bird, deeming it to be the worst birthday ever.
Percy reminds Grover of his situation and the satyr, now focused as the celedon is imprisoned, plays a tune that summons a rope for Percy. Upon noticing the sun starting to set, Percy states that they have to meet Apollo at the Empire State Building, but Grover suggests for him to put his pants back on. They arrive at the lobby where Apollo literally brightens upon seeing them. He exclaims that he'll have Hephaestus fix the celedon, but notices the lyre's scratch. Percy intervenes and tells him to have Hephaestus fix it causing Apollo to lighten up. He invites the two to watch his concert, but Percy lies, claiming they're not worthy of his music and might explode. Apollo buys the excuse and leaves with the celedones for his concert in a flash of light. Grover suggests picking up Juniper at Prospect Park to attend the sing along and eat s'mores at Camp Half-Blood. Percy winces at the mention of the sing-along, but agrees to go for the s'mores, claiming that they may still have time to turn Grover's birthday around.
- The chronology of Percy Jackson and the Singer of Apollo was difficult to place. According to Rick Riordan, the story is set in between The Last Olympian and The Lost Hero. Annabeth is named as Percy's girlfriend, which would support Riordan's claim that it is after The Last Olympian, although the story can still be easily set after the Heroes of Olympus series as well. The time however does not directly affect the storyline.
- If truly set before the events of The Lost Hero, then Percy's blessing from the Styx is seemingly not acknowledged in the story, perhaps as a way of avoiding lessening the dramatic tension of the plot. Although the story does not directly contradict the fact of his possession of this gift, Percy's actions come off no different to how he would act in combat in the time when he didn't have the gift. This is similar to how the story The Staff of Hermes (which is also set in between the two series) treats his gift, which, outside of one reference, does not seemingly affect the plot in any meaningful way.