Anybody found what they do? I found two of those as loot but they don't appear in my inventory (also checked castle inventory) and I don't get the ability to cast the corresponding rituals.

Yup, just simple rituals to do, but the only difference is how many 'uses' they have - normally you can use them over and over again. In case of rituals which were taken from quest/dungeon you get only little number near them, showing how many uses are left.

The "ritual scrolls" don't appear anywhere as inventory items, either. You'll see them in the appropriate "cast a ritual" menu, and that's it.