Tanvir stole For The Advancement of Scrying well before the player characters entered the picture, by sneaking into Eleri's home and swapping the covers with another book that was the same shape and size. He might have gotten away with it too, ironically, if Jagal hadn't stolen the decoy book himself. It's true that Tanvir has no interest in scrying, but he is interested in the Forge of Fate, as his notes betray.
Tanvir swapped the cover of this book with Advanced Blood Magick, and took it with him to research.
Eleri's seen no end of trouble from Tanvir ever since they moved to Lostmoor, but it hasn't yet escalated to actual property theft until now. They actually know quite a bit about the Forge of Fate, but refuse to tell Tanvir, instigating his little switcharoo as a way to get around their back.
Jagal broke into Eleri's house a few days earlier and stole what he thought was For The Advancement Of Scrying, in order to sell his bounty hunter services back to Eleri. Since he's illiterate, he had no idea of Tanvir's switcharoo until Lucya pointed it out.