Pre-orders of books from Type-Moon Manuscript and Avalon le Fae are available online from Toranoana and Melon Books. These two books are manuscripts and materials from Takebouki, which is Takashi Takeuchi and Nasu Kinoko’s doujin circle. While they are doujin books, they have canon material, as well as behind-the-scenes information from properties such as Tsukihime and Fate/Grand Order. Other books, including Type-Moon Manuscript, are physically available at Comiket 100. [Thanks, Famitsu!]
They are available via Toranoana and Melon Books. Though Toranoana does not inherently have international shipping (meaning you will need to get it through a proxy service if you’re outside Japan), Melon Books does. Type-Moon Manuscript costs 1650 JPY (around $12.35) and Avalon le Fae Synopsys costs 550 JPY ($4.10).
Type-Moon Manuscript is under the Tsukihime category. But it also encompasses other related works Type-Moon released afterwards. The book has information on the characters or settings from Tsukihime and related works. The information did not make it into the final product, but is canon nonetheless. There are also drafts, as well as a script of a drama CD.
As for Avalon le Fae Synopsys [sic], this is a book that summarizes the plot of Lostbelt 6 prior to when they started working on it. Aside from information that will help to answer some questions from the actual game, you can see some of the initial drafts and ideas. Avalon le Fae Synopsys is made-to-order, which means that you will not be able to purchase it at Comiket 100.
Avalon le Fae Synopsys and Type-Moon Manuscript are available for pre-order online until August 21, 2022. Other “official” items you can purchase at Comiket 100 include Touhou 18.5: 100th Black Market.