St Teresa Margaret didn't leave us an autobiography in exact detail, which is perhaps why she isn't better-known. However, we can gain a partial understanding of her from this, which is Fr Gabriel's painstaking study of her canonization documents and summary of her spiritual life.
What we discover there is that St Teresa Margaret was attracted by love to the Sacred Heart, and penetrated into the mystery of God through this devotion. She also practiced heroic virtue as she underwent severe trials in faith. Fr Gabriel explains how these went beyond the typical trials in the Night of Spirit. St Teresa Margaret verifies the doctrines of St John of the Cross through her life, despite never having read him (although her confessors surely did). Also interesting to note is how she was called to the Discalced Carmelites, or more precisely, who calls her.
Fr Gabriel notes that her limited writings read exactly like St Therese, but with a slightly different angle on her spiritual doctrine that was more particular to her own life. Unlike St Therese, since these doctrines are not explicitly presented, they would be very hard to understand and apply without something like Fr Gabriel's work. He did a wonderful job.