Side stories featuring the two protagonists of Shuuten Unknown.

Volume 8-9 is a special one, as it talks about Rapunzel's side story.
This is an additional volume called Koori no mamono no monogatari Gaiden, which is an anthology of short stories featuring different protagonists of Koori no Mamono no Monogatari,

Story 1: Cinderella (Boy) Probability & Sequel
From Essence of Purity:
At 23, Nagamori Kasumi is the new employee of a model agency but his cute face makes him look much younger than his real age. The most famous Japanese model, Nishimuro Shiki, works at the same agency... He is the so called Cinderella Boy. But he is in fact an 18 year old kid with a pretty harsh personality who drives all of his assistants away one after the other. Having enough of his constant whims, the manager tells him to choose up his own assistant. And Shiki decides to pick Kasumi...
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Story 2: After the Rain Comes the Sun & Extras
Hiyama pays a long haired boy for sex, the boy declares the rule that no real names can be used but Hiyama falls in love with him. That's why the next time they meet Hiyama refuses to pay and the boy is insulted and runs away before Hiyama tries to explain.

The first part is an older style artwork of mangaka.

From Storm in Heaven:

Toda Yuji and Kojima Isagi used to be close friends when they were little, but they drifted apart with the years. Yuji's your typical model student, while Isagi is a problem child. Even though they're almost like strangers now, Yuji keeps dreaming about a traumatic experience he had long ago, where he was saved by Isagi. After a chance encounter at school, they start hanging out with each other again. But one night they both get into serious trouble.

The White Jewel Pin is an incredibly rare, precious item. When it breaks in half, two men from different organizations gain possession of the two pieces. Each is determined to get the other half for himself: one for the sake of his own freedom, the other for the sake of the master who saved him.

From Storm in Heaven and one of their legacy groups, Bamboo Feathers:

Deep in a cave lies a demon trapped in ice who will eat any human who dares enter. The demon, Blood, waits for the human who will cry the purest tears, for the gem formed from those tears will break the spell that binds him.

One day, a young man named Ishuca appears in the cave—and knowing that his weak heart will not last long, he has come to die. Touched by Ishuca's gentle nature, Blood gradually begins to change from the cruel, ice-cold demon he once was.

Note that the artbook contains a new Koori short story.

Rakan lives alone, and the plants in his garden grow so thick and fast that it seems like a jungle. One day a man, holding a gun made of wood, falls right down into his garden. He seems to come from another world, and he's searching for someone who can bring back the green into his own dark and inhospitable world.