The 10th ghost that you're supposed to find for the lore book Luna's Lost in the dungeon should be sold by Eris (and that's the case for people that have managed to get this), but Eris isn't selling anything to me. I presume it's somehow tied to me searching all the 9 other ghosts before this part, and/or possibly related bug with the lore book itself where it completed with 10/10 after the 9th ghost despite obviously missing one entry. Friend of mine who just got the 9th one had Eris sell the 10th ghost normally afterwards.