Structured Analysis and Systems Specification
Structured Analysis may best be studied using the book Structured Analysis and Systems Specification, which may be bought on Biblio.
It cannot be found on libgen.is at the time of writing this, though perhaps someone will put it up after my writing of this by completing a book scan and putting it up.
In considering large systems, it will change your life, and teach you Structured Design skills too. Furthermore! Learning Structured Analysis will teach you general planning and life skills better than any other way, if you understand it's implications.