The Holy Thief by Ellis Peters

In the chill, rainy autumn of 1144, two
groups of visitors seek the hospitality of the Abbey of St. Peter and St.
Paul, and Brother Cadfael fears trouble has come in with them. Among the
first arrivals is Brother Tutilo, a young Benedictine with a guileless face
and—to Brother Cadfael’s shrewd eyes—a mischievous intelligence. The second
group, a ribald French troubadour, his servant, and a girl with the voice of
an angel, seems to Brother Cadfael a catalyst for disaster.

All of Cadfael’s fears become manifest as rising floodwaters endanger the
abbey’s most sacred relic, the remains of Saint Winifred. When the bones
disappear and a dead body is found, Brother Cadfael knows carnal and
spiritual intrigues are afoot. Now, in a world that believes in signs and
miracles, Brother Cadfael needs his prayers answered—as well as some heavenly
guidance to crucial clues—to catch a killer hell-bent on murder.