Constellations make sense intuitively when you see them in person, but are famously difficult to describe since they work from a place beyond words. These are some of the most helpful introductory materials available about the history of constellations, how they work, and what you can expect when you encounter the process for the first time.
Intro to Constellations Books
Some of the most recommended books for an introduction to constellations.

Acknowledging What is: Conversations with Bert Hellinger
Bert Hellinger, Gabriele Ten Heovel

The Constellation Approach: Finding Peace Through Your Family Lineage
Jamy Faust and Peter Faust

The Healing of Individuals, Families & Nations: Transgenerational Healing & Family Constellations
John Payne

The Language of the Soul: Healing with Words of Truth
John Payne

It Didn't Start with You: How Inherited Family Trauma Shapes Who We Are and How to End the Cycle
Mark Wolyn
Family Constellations
Family Constellations was first popularized in Germany, in the aftermath of WWII. Some of the first discoveries were made with German families, who were looking at the after-effects showing up in the descendants of victims and perpetrators in the war. This article explains the origins, why there is often historical undertones in constellations, as well as how they are set up in a group process.
Businesses and Organizational Constellations
After seeing how constellations worked in a family system, early practitioners began extending the work to look at a wider range of systems, including businesses and organizations. Systemic constellations is now known as an effective tool for improving businesses. Jan Jacob Stam is one of the foremost practitioners and trainers for how constellations works in management, non-profits, and government.Intro Video Courses
For those who are ready to dive a bit deeper, one of the first generation of constellators, Bertold Ulsamer, offers very a clear and practical video introduction to constellations. Many of them are free.