Given that we all have an obligation (as stated in the mishna berura) to have a regular mussar seder everyday, is one allowed to fulfill that seder by “learning” a secular book (often termed “self-help” books) if he feels that it is really helping him grow and improve his middos?
Yes, this is certainly fitting.
The concept of a “mussar seder” is to focus on self-growth that will help a person in his Avodas Hashem.
If you sense that these books are conducive to your personal growth, it is fine to use them.
At the same time, it is good to use the conventional mussar books too, because of the Torah content