I've discovered a situation in which a villager will not take on a job.  He is the only unemployed villager in the village.  I put down lecterns until they stopped creating happy sparkles (around themselves and the existing librarians).  This villager just won't bite.  He's not a nitwit: he's not in green and he follows the same sleep schedule as the others.  It doesn't seem to be a pathfinding issue.  His bed is right there.  There's no apparent explanation.