PC Somehow lost a Black Book?

[Seven]

I somehow lost a black book.. (The Sallow Regent)
This happened long ago without me realising it, so I no longer have a save I can go back to.

Anyway, I've cleared the White Ridge Sanctum, and removed "The Sallow Regent" a while ago, but I skipped it and went out of Apocrypha again.
Then who knows what I did because now it's not in my inventory, and not in White Ridge Sanctum.

Is there any cheat codes or whatever I can use to get it back?

I'd really appreciate any help..

 

[deltamind]

Between playing the old character and the new one, did you add or remove any mods?