Read the trilogy first, then the optional companion:
Macmillan labels The Mirror of Infinite Endings both a companion and Volume 4. Shelf of Worlds places it after A Curse for True Love.
For the core trilogy alone, stop after A Curse for True Love.
Sources: Macmillan — Once Upon a Broken Heart series, Flatiron Books — The Mirror of Infinite Endings / 9781250462039 · Verified Sep 20, 2026
Publisher labels this Book a companion / Volume 4; Shelf of Worlds places it after the core trilogy without making it mandatory.
Short answers, with sources.
Read Once Upon a Broken Heart, The Ballad of Never After and A Curse for True Love in that order. Shelf of Worlds then places The Mirror of Infinite Endings after the trilogy as an optional companion.
Sources: Macmillan — Once Upon a Broken Heart series, Macmillan — Once Upon a Broken Heart, Macmillan — A Curse for True Love, Flatiron Books — The Mirror of Infinite Endings / 9781250462039
Macmillan labels it both a Once Upon a Broken Heart companion and Volume 4. Shelf of Worlds places it fourth after A Curse for True Love while keeping it optional and associated rather than making it a fourth core trilogy novel.
Sources: Flatiron Books — The Mirror of Infinite Endings / 9781250462039
These sources support the reading order and important details about the books and their editions.