For further information on St John of Damascus, particularly regarding background to his chapter from his Fountain of Knowledge on Islam as a heresy, see especially Daniel J. Sahas, John of Damascus on Islam: The “Heresy of the Ishmaelites” (Brill, 1972). Sahas provides a great deal, pretty much everything we can piece together actually, about the life of St John of Damascus, his family (his grandfather was responsible for turning over Damascus to the Muslims, for which he was hated, and his father was a high-ranking mininster as well), his profession before he left Damascus to become a monk at St Sabbas near Jerusalem (he was πρωτοσυμβουλος to the Caliph, the highest ranking position in the Muslim empire after the Caliph himself), the reaction of the iconoclasts to his writing, and so on. He includes the Greek text from Migne’s Patrologia Graecae for the chapter on the Ishmaelites (I used this text after having corrected it according to the Thesaurus Linguae Graecae text taken from P. B. Kotter, Die Schriften des Johannes von Damaskos, vol. 4. Patristische Texte und Studien 22, Berlin: De Gruyter, 1981) and that of the Disputation of the Saracen and the Christian, traditionally attributed to St John, both with translations.
St John’s Fountain of Knowledge is a vast work in three parts: The Philosophical Chapters, in which he presents various aspects of the height of secular learning of his day; On Heresies, the first 80 chapters of which are taken verbatim from summaries of the heresies in the Panarion of St Epiphanius, and the last 23 by St John; and The Exact Exposition of the Orthodox Faith, which is essentially a catechetical manual, and is more than twice the length of the above two sections combined. The entire Fountain of Knowledge is available in a handy English translation in the Fathers of the Church series, fortunately much more affordable than the above-mentioned volume.