Hey there, when you complete a ritual set, the Tome adds itself back to your inventory as an empty version of itself. This works fine except for when you are in an activity where loadouts are locked - something about this means the empty Tome will fail to add itself back to your inventory, causing it to go missing. If you go back to the Shaping Slab at Eris' apartment, you can reclaim it back from the place you picked it up in the first place, rank 3 of the Path of Resolve reputation rewards. Note that if you're on Path of Ambition you'll need to swap it back to Resolve first.