I would imagine that Laura's mother was the only person absorbed by the priest, specifically--and that she was sealed in that book because the priest was using her latent magics to bolster himself, thereby requiring that he "keep her close at hand". I went back through to re-read, and saw no mention of the priest taking others beyond the mother (and attempting to take Laura herself), so I think the mom was the only one he dealt with personally.
So when he died, the demon dispossessed his body, the book lost its magic, and she was unsealed--allowing for her to return to this reality/dimension/what-have-you and be rescued by Kachua & Co.But, the others purportedly absorbed by the demon were done so independently of the priest, and therefore are "kept" elsewhere, perhaps in other objects/phylacteries/etc. That would explain why Laura's Mom came out of the unsealed book all on her own.
There might be other humans/beings like the priest that are also powered-up by absorbed humans that they'll have to deal with as well--since they defeated the transformed priest, but not the demon who'd been making contracts with him / Laura's Mom / others.