This book is pretty good for a fundamental understanding.
https://www.amazon.com/SAE-Evolved-Packet-Core-Revolution-ebook/dp/B004CFAF8Q
What do you mean by "domain part"?
Vendor documentation is also one of the main resource on how things work as it is usually describe traffic flows and procedures on a pretty good level. Of course it depends on vendor.
3GPP specifications is also a good source, but not so easy to read at the beginning.