The Book of Memnarch
The Book of Memnarch was written by the priest Tovias Memnarch in CY __. It is a unique part of The Cycle of the New Sun. It is slightly different and outside of what is normally taught in the canonical Records of the Church of Light. The styles are unique and structures a bit alien. And some of the information contradicts canonical records.
This book details the rituals and materials used to summon the Angels of The Radiant Spirit. It details the failed attempts and successes. Each success details the angels summoned, except that it details one angel, Sauvabraeth, also known as the Light of Truth, as the first successfully called forth that is unmentioned in the official chronicle.
The Crystal of the Covenant
It also details something left out of the canon chronicles that claim each angel was given a Crystal of the Covenant. The Lightbringers placed a tiny portion of their Essence and bound the angels to it. The angels carry this crystal to more easily channel their divine power.