Internal family systems therapy by Richard Shwartz was a very interesting read. Its available on lib.gen.
Here is the amazon link :
https://www.amazon.com/Internal-Family-Systems-Therapy-Guilford/dp/1572302720
Internal Family Systems is a theory that human minds consist of parts that have their own independent will and drive. Yes it's also useful as a therapy model, but it's not a symptom of illness.
This book gives the best overview of the system
https://www.amazon.ca/Internal-Systems-Therapy-Richard-Schwartz/dp/1572302720
And here's a brief description of the theory
https://www.selfleadership.org/outline-of-the-Internal-family-systems-model.html
Hope this is useful