Unbroken in Time. The After Cilmeri Series, #15

In 1282, David and his sister Anna crashed through time to save the life of their father, the last Prince of Wales. Thirteen years later, David is a grown man-husband, father, and High King of Britain. He is also the Duke of Aquitaine, a fact which King Philip of France appears to resent. So when Philip summons David to Paris to bend the knee for the right to remain Aquitaine's duke, David is the first to suspect treachery.
And when he learns Philip is taking advice from Guillaume de Nogaret, mastermind behind the annihilation of the Templars, the expulsion of the Jewish community from France, and the assassination of the pope, he's sure of it. With the throne of Aquitaine, France, and even England on the line, David must call upon eight hundred years of history only he knows to prevent Avalon's past from becoming Earth Two's future ... Unbroken in Time is followed by Outcasts in Time. Complete Series reading order: Daughter of Time, Footsteps in Time, Winds of Time, Prince of Time, Crossroads in Time, Children of Time, Exiles in Time, Castaways in Time, Ashes of Time, Warden of Time, Guardians of Time, Masters of Time, Outpost in Time, Shades of Time, Champions of Time, Refuge in Time, Unbroken in Time, Outcasts in Time, Hidden in Time, Legacy of Time, Renegades in Time.
Also, This Small Corner of Time: The After Cilmeri Series Companion.