The Fifth Hawkenlye Mystery, featuring Abbess Helewise and Josse d'Acquin.
An old man dies in the pilgrims' hostel at Hawkenlye Abbey. Nothing strange about that, he was ill – probably why he was there in the first place – except that the youth accompanying him disappears that same night. Then the rotting corpse of a much younger man is discovered in the forest. This one had clearly been murdered, and Abbess Helewise begins to get upset.
Meanwhile, Sir Josse d'Acquin has a visit from Prince John (he who would one day be known as Bad King John) and his "Magister", who turns out to be the seer and sorcerer John Dee. They are searching for a certain Galbertius Sidonius, who, they believe, will be trying to contact Josse. Why? They do not say, and we are not told – yet.
Part 2 is a beautifully written flahback, from Josse's time in 1192 to that of Josse's father, Geoffroi d'Acquin. In 1148, Geoffroi set out on the Second Crusade, during the course of which he saved the life of a small boy and was subsequently presented with a mysterious and magical heirloom, the sapphire known as the Eye of Jerusalem. This, as you may have guessed, is what Prince John is really after.
Finally, in Part 3, we return to the present, 1192, where deep in the forest, among the mysterious and pagan Forest People, on the night of Samhain, Josse's Joanna (see The Tavern in the Morning) is about to give birth to his child.
Very highly recommended – as is this whole series!