Good points -
I like Max. He's a good guy with a true calling who doesn't judge others. For a not necessarily "Christian" mystery, I think it's better at presenting religious lessons than some of the preachier ones. He makes sense as a detective, too, since he's former MI5. It's reasonable that the bishop would send him to investigate.
The abbey is a good setting and the nuns are an interesting lot. Each has a past, but those don't count anymore, since they've joined the sisters. The nuns are walking a fine line between being separate from the world and making money by selling their products and having guests stay.
Max's relationship with his pregnant girlfriend, Awena, stays in the background. I was afraid it would dominate this one after the end of #3, but it didn't. Awena's a great character, very in touch with nature and the seasons, but I don't necessarily like an overly complicated romance to intrude on a mystery.