If you are stuck, you can always refer the walkthrough (there is a link to it in the game's folder, CTRL+F for "ritual spell", and you should find what you need).
The easiest spell to find for this is in your house, actually (detect magic in your room, and then interact with the mirror), but that might take too many time slots to learn, and since this quest has a 3 day limit, I am not sure if you can go for that one.