Juice Like Wounds by Seanan McGuire
This is the story of three girls who came and went into the Market. Moon, the youngest, was almost always there and was left by her mother in the care of the Market. One, the oldest, was Moon’s first friend, Mockery, who came to the Market through a series of doorways, leaving everything behind including her true name. One, Lundy, traveled back and forth to the Market almost as often as she pleased, until she couldn't do it anymore. Both Mockery and Lundy were friends with Moon, and on one trip, they all three became friends, if only for a short while. Lundy provided pies for their lunch, Mockery traded crawfish for lemonade and Moon traded stones for candy, and their friendship began.
The three decided that they would become the heroes of the Market and would keep it safe from monsters and be paid in fair value for 100 years, but they had to find a monster to slay in order to make that true. Lundy spent a lot of her time in the Market with the Archivist, reading her books and asking her questions, so one time, she asked her about people who don't believe in the fair value of the Market and decide to leave. The Archivist told her that there are some that leave the Market, and the world, all together, and some don't have anywhere else to go, so they live in the outskirts outside the Market where they owe no fair value to anyone. Some are fine on their own and find peace, but some, the worst ones, become monsters, and that good children shouldn't mess with them because they're is always a cost.
Lundy told her friends about the monsters and they decided that, if they defeated one of these monsters, it would be like setting things right in the world and would not incur them any debt but instead it would pay off an old one, and that's a good thing! But where do they find a monster? Mockery suggested looking in the Archivist's books to find a place in the Market that's not mentioned anymore, but one that stopped being mentioned recently, because that place would have the newest and most easily defeatable monster, and Moon and Lundy agreed.
Lundy started reading books the next day and found one that mentioned a pomegranate grove, but Lundy had never seen pomegranates for sale in the Market, so she asked the Archivist about it. The Archivist rarely made mistakes, but she did this time, and told Lundy that the grove was to the West. She did not tell her, though, that inside the pomegranate grove was a young girl called Zorah who came from Lundy's own world and whose heart was filled with so much venom that when she stopped paying fair value in the Market and left, she did not become a bird like all the others, but instead became a wasp. The Archivist didn't tell Lundy anything about the monster, but she did ask her to find and enjoy another book, she also asked her not to go alone to the pomegranate grove.
Lundy found her friends eating and picking blackberries and told them that they would need to travel West to find the grove so they stopped and prepared to set off. Lundy had a knife she used for scaling fish and Mockery had a spear and Moon had a sling. They took their gathered berries to trade for a nice blanket and some vegetables and set off. They should have waited, but they were on a quest, so they did not.
They traveled West through the woods and finally came upon the grove. Moon, being the youngest and most foolish of the three, rushed to gather the pomegranates that had fallen to the ground, because after all, in the Market, fallen fruit belongs to no one, so why shouldn't she take some? As she reached for the fruit, a large wasp the size of Lundy flew in, and Mockery dove to protect her friend. Lundy stood up to the monster and told her they needed the fruit and that she wasn't giving fair value, and the Wasp countered that children shouldn't be working to earn anything, but then she's struck in the wing from a pomegranate thrown by Moon and her sling.
The Wasp Queen, furious, turned toward Moon and flew to attack, but again Mockery dove in to protect her friend, though this time she held her spear aloft. As Mockery’s spear pierced the belly of the Wasp Queen, the Wasp Queen’s stinger pierced Mockery's belly. Moon sat terrified while Lundy jumped in with her knife and cut the Wasp Queen’s head from her shoulders. Unfortunately, as she died, the Wasp released her venom into Mockery who also died. Rather than sit and mourn their friend, Moon gathered pomegranates and the Wasp’s head to pay for Mockery's funeral while Lundy carried her body. Lundy and Moon made their way back to the Archivist who was not very surprised that all three girls did not return from the pomegranate grove. They buried Mockery by the river, and then, the next day, Lundy went through her door and back to her own world, leaving Moon all alone.