[I’d say this is a bit of a curate’s egg. It’s extensive, and free, and brings in several highly rated systems authors to give approachable, light summaries of their methods and approaches. You can see the editorial team here: https://www.scienceopen.com/collection/91ba7261-ecaf-4e92-ab55-2e9cfce483d0 under ‘about the journal’, and under ‘open peer review’ can see their ‘unusual’ peer review process]

They say:

This Handbook details the theory and practice of Systems Thinking in the areas of social systems, management, and policy. The contributing chapters from numerous authors show the diversity of the field, and this first chapter seeks to identify patterns in that diversity to demonstrate an underlying unity among the plurality of methods, approaches and interventions throughout the field.

Publications – The Handbook of Systems Thinking – ScienceOpen

https://www.scienceopen.com/search#collection/4afbd5c3-ca08-405f-89cd-9f919db2c8e8