Our ancient sources include:
Cassius Dio’s Roman History (Earnest Cary translation)
The Historia Augusta (David Magie translation)
Catullus: Cybele and Attis
Phlegon of Tralles’ Book of Marvels (William Hansen translation)
Our modern sources include:
Marilyn B. Skinner: Sexuality in Greek and Roman Culture
Sara Barish: “Iphis’ Hair, Io’s Reflection, and the Gender Dysphoria of the Metamorphoses.” Eidolon.
The Queer Classicist: Famed for His Deeds, Though Born Female: Ancient Evidence of Transgender Lives
Luoyao Zhang: Case Study: Transgender Men in Lucian’s Dialogue of the Courtesans
Livius.org: The Religion of Cybele
BBC News article about the grave of a trans woman (warning: title uses an offensive term for trans people)
Siobhan Ball: “The Chill Roman Priests Who Worshipped a Goddess and Castrated Themselves.” Vice.
Greek Legends and Myths: Caeneus in Greek Mythology