DISCLAIMER: THIS GAME CONTAINS PORNOGRAPHIC SCENES FEATURING A CHARACTER DRAWN TO LOOK LIKE A CHILD - IF THAT IS ILLEGAL WHERE YOU LIVE OR YOU JUST FIND THAT TOO GROSS TO ENJOY THE GAME, DO NOT PLAY
Two additional short VNs or fandisks were released following the original Tsukihime - Tsukihime PLUS DISK in 2001 and Kagetsu Tohya later that year.
The latter, Kagetsu Tohya consists of the main story - Twilight Glass Moon, Fairy Tale Princess - and a collection of short stories known as the 10 Nights of the Dream. The short stories are unlocked by viewing a relevant scene in the main story.
Twilight, Glass Moon, Fairy Tale Princess
About a year after the events of Tsukihime Shiki has settled in to a new daily life with Arcueid, Ciel, Akiha, Hisui and Kohaku all parts of it.
The main story is a look into 10 days of that new life, 10 days that seem to go on and on and on and on. Where the School Festival may or may not be coming up soon, where Akiha and Shiki may or may not be going to the same school and...
where there may or may not be a murderer in the streets.
There is a strange little girl with strange ears somewhere in town.
It's almost like she has her eyes on Shiki.
Nothing to worry about, her very presence is like some fleeting dream.
Shiki should just enjoy his new, ever-changing life, and try not focus on why it seems so malleable and ephemeral.
The world she was born into was so small.
Just a magus, a windmill and a sunset.
The end of the world is coming...
Ten Nights of the Dream
As the name suggests, 10 short stories can be unlocked by playing through the game. Due to the fact that one action in one loop can overwrite choices for the next, I'd reccomend playing with a guide if you want to unlock all the stories.
The stories range from fun out-of-continuity stories like "Good Luck, Ciel-Sensei", "The Tohno Family Con Game", and "Imogirisou" to the in-continuity but still light "Nanako-Chan SOS" and to the more serious "Dawn" to "A Story for the Evening" serving as the sequel to the true ending of the Akiha Route to important character backstories like "Crimson Moon" (For Roa) and "Red Demon God" (For the Nanaya clan and Tohno Makihisa) and even to just porn like "Flower of Thanatos".As to whether or not "Hisui-chan, Inversion Impulse!" is canon to her good ending is beyond me.
The stories:
Good Luck, Ciel-Sensei is a largely comedic story about Ciel-Sensei quizzing the main cast and her attempts to start a relationship with Shiki.
A Story for the Evening; Set after Akiha's good ending, it is told from Akiha's perspective as she tries to navigate moving back to her boarding school, solving the mysery of the cursed post boxes and deal with the disappearence of her brother.
Crimson Moon deals with the initial meeting between Arcueid and Roa from Roa's perspective and features a rare appearence from the "Crimson Moon Brunestud" personality within Arcueid.
Red Demon God; told from the perspective of Nanaya Kiri - Shiki's biological father - , it deals in more detail with the growing feud between the Nanaya and the Tohno clans and the destruction of the Nanaya. Notable for being the only appearence of the real Kouma Kishima - the killer of Shiki's family - across the original Tsukihime timeline.
In Nanako-chan SOS Arihiko has to deal with an unwanted new roommate, the spirit Nanko-chan.
Imogirisou is one of two parodies of the horror sound novel Otogirisou using characters of another series I know, the other being Kirigirisou, featuring Kirigiri from Danganronpa (I'm pretty sure she'll die if it's a horror game). It's a comedic take on a hauted mansion story.
Here's the box art of Otogirisou for comparion's sake:
Flower of Thanatos is a short porn story where Shiki, Hisui and Kohaku have a threesome. Not much too it as far as I'm aware (I got bored and space-bar-ed my way through it.
Hisui-chan Inversion Impulse takes place after the Good Ending of the Hisui route but may or may not actually be canon to it - it's another comedy story where Hisui... inverion impulses.
The Tohno Family Con Game is a game where the Tohno Family (which apparently includes NRVNQSR Chaos) and Satsuki play zombie-tag.
Dawn is actually offically published fanfiction written by Maruyama Kugane and deals with Shiki waking up one night with a thirst for blood. After blacking out, he wakes up to a mess of gore. While Ciel believes it to be the work of Arcueid, Shiki isn't so sure that he wasn't responsible and Arcueid's not so sure Shiki is Shiki.
What makes the various short stories worth your time is the look into what the characters other than Shiki are up to and a chance to see the expanded lore of the Tsukihime world.
Want to see Roa's perspective? Akiha's? Shiki's biological father? Well, here you go.
Want something light and comedic? There's plenty for you to chew on.
Want to finally see characters only mentioned elsewhere? Nanako-chan, Crimon Moon and Red Demon God will be right up your alley.
Not to mention, two stories give explicit depcitions of what happened after a particular route instead of trying to keep it vague like most Type-Moon media does.