Alamut Series by Judith Tarr

A knight from across the sea, a beleaguered kingdom, a spirit of fire with a deadly secret - Judith Tarr's beloved novel of grand romance and high magic in the age of the Crusades appears for the first time in digital form. Join Prince Aidan, son of a mortal king and an immortal enchantress, and the deathless Assassin Morgiana, in a saga of war and truce, betrayal and honor, hate and love.

In the years before the Third Crusade, Price Aidan of Rhiyana goes to the Holy Land. The younger of twin sons born of a mortal king and an Elven woman, he is heir to magic and said to be the finest Christian knight alive. Yet he pledges his heart to immortal Lady Morgiana, a steadfast Muslim and assassin. Sequel to Alamut.