If you want to know more about Justinian II then I'd check out the main historiographical sources on his reign from an eastern Roman point of view, namely, Theophanes the Confessor's Chronicle (with an English edition translated by Harry Turtledove), and the Short History of the Patriarch Nikephoros I (translated by Cyril Mango). Also there is the Canons of the Quinisext Council 691/692 (translated by Richard Price) which cover decisions and acts of the Church council summoned by Justinian II to discuss the Fifth and Sixth Ecumenical Councils. Besides this you could also check out the partial translation of Khalifa ibn Khayyat's Tarikh (translated by Carl Wurtzel as History on the Umayyad Dynasty 660-750) and vols 21 and 22 of the English translation of al-Tabari's work, both of which can offer a Muslim point of view on Justinian II's reign and his interactions with the Umayyad Caliphate.