What is the motivation for trace class operators? Can any body suggest the most general and standard reference that icluse schatten p class operators as well. I have following references

1. Operator theory by Conway
2. Opeator theory in function spaces by Kehe Zhu

Can any body suggest even better?

What is the motivation for trace class operators? Can any body suggest the most general and standard reference that includes Schatten p class operators as well. I have following references

1. Operator theory by Conway
2. Operator theory in function spaces by Kehe Zhu

Can anybody suggest even better?