"Internal Family Systems is a powerfully transformative, evidence-based model of psychotherapy. We believe the mind is naturally multiple and that is a good thing. Our inner parts contain valuable qualities and our core Self knows how to heal, allowing us to become integrated and whole. In IFS all parts are welcome.
IFS is a movement. A new, empowering paradigm for understanding and harmonizing the mind and, thereby, larger human systems. One that can help people heal and helps the world become a more compassionate place." (https://ifs-institute.com)
You can find a lot of resources on the Resources page of the IFS Institute. However, I think this is the most useful introduction to IFS: Richard D. Schwartz, Greater Than The Sum of Our Parts: Discovering Your True Self Through Internal Family Systems Therapy (available for free with a new subscribtion to Audible - and you can cancel anytime, but if you are already subscribed to audible, I think this audiobook is the best use of you monthly credit.)