>>4425458
I'm really glad you enjoyed her story in the Fate route! She's absolutely amazing, and I've loved her for nearly 17 years now.
I highly recommend reading Garden of Avalon, which is her backstory before Fate/Zero, discussing the Type-Moon version of the Legend of King Arthur. Its a relatively short read, but is very important to her story in FSN, Zero, and various chapters of FGO (Camelot, and Lostbelt 6, specifically). It's a relatively short read at about ~70 pages long (counting illustrations). It was released with the Unlimited Blade Works Blu-ray box set. It was written by Kinoko Nasu, so it's the most canon version of her story regarding her life before becoming a servant. Here is a fan translation:
https://www.reddit.com/r/fatestaynight/comments/repky2/garden_of_avalon_novel_complete_translation_in/