Two additional short VNs or fandisks were released following the original Tsukihime - Tsukihime PLUS DISK in 2001 and Kagetsu Tohya later that year.
The former, Tsukihime PLUS DISK consists of the short story Alliance of Illusionary Eyes - a story not told from the perspective of Shiki but of . In addition, it contains the first 4 chapters of Kara No Kyoukai, various websites as html, some older versions of Tsukihime and some bonus images.
Alliance of Illusionary Eyes is a told from the persepctive of Seo Akira, a girl with Mystic Eyes that allow her to predict the future. After a public incident where she tells everyone around they shouldn't commit a murder as seen in her prediction, she and Shiki team up to prevent the murder and protect her from the murderer.
No CGs sadly so all you have is the text description.
Outside of that it contains a even shorter largely comedic story named "Geccha" - an out of continity story where the various heroines of Tsukihime discuss their roles in the story and some trivia.
In the Tsuki-bako re-release a second Geccha where Satsuki and Neco Arc discuss Satsuki's cut route and the original Melty Blood, showing off designs for Sion and Wallachia, and Kinoko's Masterpiece Experimental Theatre which is complete nonsense story.
Ultimately, Tsukihime Plus Disk is just a neat add-on to the original, it doesn't offer too much but why skip over it?
Plus Disk Bonus Wallpapers - to see artists hover over, various spoilers for Tsukiime
If you don't recognise the girl with white/grey hair, that's okay because she's from Kagetsu Tohya - these CGs were only added in the Tsuki-Bako re-release.
Plus Disk Bonus Images - Explicit Images Included
As for their origins, I can say that the first and eighth I believe are sourced from the original trial edition of Tsukihime, that the seventh and the eleventh are from the half-moon edition (with the latter being packaging) and that the second is from the Tsukihime BBS and was based on various typos made it look like Hisui was trying to convince Shiki he was the killer in the town in earlier editions leading to the in-joke that Hisui was attempting to hypnotise him into murder.
That itself would become the basis of the "Brainwashing Detective Hisui" parody character (basically her version of Ciel-Sensei, Neco-Arc or Doctor/Magical Amber), a brilliant dectective that always solves the crime by brainwashing the person she thinks did into doing it in the first place.