Nearly everything told about Apollonius comes from a single book: the Life written by Philostratus around the 220s, at the commission of the empress Julia Domna, some one hundred and twenty years after its subject died. Philostratus claims to be working from the memoirs of a disciple, Damis of Nineveh; most scholars regard Damis as a literary device, and Maria Dzielska's study concludes that a real ascetic Pythagorean teacher underlies the book but that the travels to India and Ethiopia, the miracles and the confrontations with emperors are largely composition.
What is reasonably secure is a first-century wandering Neopythagorean: vegetarian, celibate, linen-clad, a reformer of temple cult and a critic of blood sacrifice, who taught in the cities of Asia Minor and had a considerable posthumous reputation.
The afterlife is more interesting than the life. Because Philostratus's Apollonius heals the sick, casts out a demon, raises a girl apparently dead and departs without leaving a body, he was deployed in late antiquity as a pagan counter-Christ, explicitly so by Hierocles around 300, which drew a book-length reply from Eusebius, and Christian polemic thereafter treated him as a sorcerer. In the Arabic and then the Latin magical tradition he became Balinas, master of talismans, and had a substantial body of Hermetic and talismanic literature attributed to him that he certainly did not write, including versions of the text underlying the Emerald Tablet. The occult Apollonius is a construction of the centuries after him, built on a book that was itself already a construction.