[sufue]

The Subtle Serpent is the fourth in the excellent (IMO) Sister Fidelma series by Peter Tremayne. It has dropped to $2.99 at Kindle and Kobo US. Titles in this series don't drop in price very often, in the US, at least, so grab it if you want it.

Kindle US: https://www.amazon.com/Subtle-Serpen...dp/B007NKMUCQ/
Kobo US: https://www.kobo.com/us/en/ebook/the-subtle-serpent-1

Spoiler:

In the year 666 A.D., a headless female corpse is found in the drinking well of a remote abbey in southwest Ireland: clasped in one hand is a crucifix; tied to the other arm is a pagan death symbol. Sister Fidelma--sister to the king of Muman, a religieuse, and an advocate of the Brehon law courts--is sent to investigate. En route, she encounters a Gaulish merchant ship under full sail off the Irish coast--one whose crew and cargo have vanished without a trace.

Faced with a tense local situation, Fidelma must discover first the identity of the body in the well and uncover who was responsible, then find out what happened to the missing crew of the adrift merchant ship, and, finally, determine how these bizarre events are connected. For these events are more than simply disturbing--the peace of the entire kingdom rests upon their solution.