The two most important sources for the reign of Justinian II are the chronicles of Nikephoros and Theophanes. I have read both in the Greek. Both use some older common source; each also offers information the other lacks. Justinian's modern biographer, Constance Head, downplays some of the more horrific episodes in the Emperor's second reign, episodes recorded only in the chronicle of Theophanes. I must respectfully disagree with her interpretation. The kinds of things Theophanes has Justinian doing strike me as consistent with his actions and personality as described in the Liber Pontificalis and in the Syriac chronicle of Dionysius of Tel-Mahre, to which Head did not have access (although she did use the later Syriac chronicle of Bar-Hebraeus, who draws on Dionysius's work).
I have for the most part stuck very close to the historical record of Justinian's career, which is quite sufficiently amazing without embellishment. I altered the actual events of the sixth ecumenical synod in a couple of ways, first by having Makarios of Antioch present when his fellow monothelite Polykhronios tried to raise the dead (Makarios had actually been condemned and removed from office by then), and second by having Bishop Arculf of Gaul take part in the synod. Arculf was in fact in Constantinople at the time, but as a pilgrim on the way home from Jerusalem. His native town here is fictitious, and his meeting with Justinian II is conjecture on my part. I should note here that theology was so vital to the world of the late seventh and early eighth centuries, and so intimately intertwined with politics, that a novel of this sort, which appears to place undue stress on it, in fact severely understates its importance.
Most details of Justinian's private life are also conjectures. It is, however, worth pointing out that Theodora did act in her new husband's interest and against that of her brother on very short acquaintance with Justinian, which may perhaps speak well for him in that regard.
I have followed Richard Delbra's conjecture that Justinian had his mutilated nose surgically repaired while in exile (in fact, I dare take the liberty of saying here that I made a similar conjecture myself before learning of Delbra's, which has- some- iconographic evidence to support it). Auriabedas is fictitious, but Indian surgeons at the time were in fact the world's leaders in what we would call plastic surgery, and could and did perform operations such as the one I describe Justinian submitting to. Details of the procedure are from Guido Majno, The Healing Hand: Man and Wound in the Ancient World (Cambridge, Massachusetts, 1975).
The only place Myakes appears in history is in the melodramatic scene during the storm on the Black Sea. His relationship to Justinian and his ultimate fate are novelistic inventions.