If you do a search for "how to come up with a title for your novel" a bundle of ideas emerge.
The one that really hit me was: make sure people can SAY the title OUT LOUD without stumbling over words, being embarrassed, or having people ask them, "what the heck is that?"
So, I'm mulling over the title. As you can see on the side of the site, the working title is Murder by Monstrance. Most people, I find, neither know what a monstrance is (I didn't, at first) nor find it easy to pronounce. I love the word; it is easily confused with "monstrous," which indeed is what all murder is. However, that's not going to help with selling a book, methinks.
Here are a few alternative titles I've been fiddling with:
The Murder Vessel (a monstrance is a vessel; a person can also be a vessel of something, like a vessel of God, somewhat archaically)
The Fourth Victim (not sure this tells a reader much about what's to come)
A Furious Motive (is that even grammatically correct?)
The Killing at St. Eligius (?)
Murder in the Sanctuary (bet that's been done)