I have the same bug with Aloth, Kalakoth's Minor blight causing the grimoire to disappear, I don't save often but use the quick save very often so this bug is a game breaker for me, as my previous backup is hours ago. Will wait till a patch appears before playing again, hopefully soon. As to how the bug happened, I am not sure, I think I was fighting some duid giant boss under my stronghold basement, and beat him with Kalakoth spell activated, after that Aloth is bugged.