Book Log 2021 #34: The Heretic's Apprentice by Ellis Peters

A local man once accused of holding heretical views dies during a pilgrimage, and is returned to Shrewsbury by his attendant, who was tasked with finding his master a burial site on the abbey grounds. That task grows more difficult when the attendant is also accused of heresy by a fellow servant, who fears losing his place in the household. When it becomes clear that's not an issue, the servant goes to recant his charges - but winds up dead.

Once again, it's up to Cadfael to help find the killer and prove the attendant's innocence. He is of less help against the heresy charges, which are pursued by a visiting canon. 

I liked that there was a heresy subplot to augment the usual murder mystery, it's not a topic that's come up much (or at all) in the series. It gives an extra insight into religious life and practice at the time, one that we don't often consider in the current day. Seriously, when was the last time you heard of someone charged with heresy?