Here are my two essential books, and both are online:
"Gregory the Great, Dialogues, Book 2: Life of Benedict"
The Life of Benedict was written about 593 AD, about 50 years after the death of Saint Benedict. It was, according to the New Advent Encyclopedia, “written in 593, three years after [St. Gregory’s] elevation to the papacy, at the request of certain monks of his household.”
The Life is the only source available [other than the Rule and perhaps the Rule of the Master ] on the life of St. Benedict.
"The Conferences of John Cassian"
Cassian lived 360 to 435 AD, about two generations after the end of the Roman persecutions in 313 AD. Cassian lived and toured the flowering of Egyptian monasticism during some of its greatest years. From the New Advent Encyclopedia: “St. Benedict made use of Cassian in writing his Rule, and ordered [in **Chapter 73] selections from the “Conferences”, which he called a mirror of monasticism (speculum monasticum), to be read daily in his monasteries.”
Cassian was important in the introduction of Eastern desert monasticism into the early medieval west and for the ideas used by St. Benedict in the Rule.