It would depend in what scenario you wanted to apply the concepts. I find that the best ROT (return on time) can be had by reading research papers. You need to have an understanding of the terminology or be prepared to slog through it. There are some books that have very useful information.

Below are some of the resources i have been using/used:

A brilliant introduction to systems thinking by John Sterman who is a legend in the field.

https://www.youtube.com/watch?v=AnTwZVviXyY

Limits to Growth by Donella Meadows is a classic and a great introduction to some of the concepts.

https://www.amazon.com/Limits-Growth-Donella-H-Meadows/dp/19...

My review:

https://www.goodreads.com/review/show/2231577740

Books focused on Application of the theory, with various approaches to systems thinking that can be applied to solve challenges. There is some overlap between the two books below as they both have a catalogue of methods. They are categorized differently. I would read the introductory chapters of both of these and then review the chapter layout and see which of the two i prefer.

Systems Thinking: Creative Holism for Managers by Michael C. Jackson

https://www.amazon.com/Systems-Thinking-Creative-Holism-Mana...

And

Systems Concepts In Action by Bob Williams and Richard Hummelbrunner

https://www.amazon.com/Systems-Concepts-Action-Practitioners...

Research Papers/Book Extracts:

A Definition of Systems Thinking: A Systems Approach by Ross D. Arnold, Jon P. Wade

Soft systems methodology: a thirty year retrospective by Peter Checkland

Guidelines for Drawing Causal Loop Diagrams By: Daniel H. Kim The Systems Thinker, Vol 3, No 1, pp5-6 (Feb 1992)

More at: https://thesystemsthinker.com/

A brief guide to interactive planning and idealized design by Russel Ackoff

Application of soft systems methodology to the real world process of teaching and learning by Nandish V. Patel

An Application of Checkland’s Soft Systems Methodology to the Development of a Military Information Operations Capability for the Australian Defence Force by R. J. Staker

Defining innovation: Using soft systems methodology to approach the complexity of innovation in educational technology by Glenda Cox

The viable system model: A briefing about organisational structure by Raul Espejo

Complex Adaptive Systems by JASON BROWNLEE

Sustainability, complexity and learning: insights from complex systems approaches by A. Espinosa, T. Porter

Stafford Beer’s Viable System Model An Interpretation By Trevor Hilder

Principles of the self-organizing system by W. Ross Ashby

Guidance on applying the viable system model by Sandra Hildbrand Shamim Bodhanya

Embracing Human Experience in Applied Systems-Thinking by José-Rodrigo Córdoba-Pachón

The Need for a Systemic Approach to Change Management—A Case Study by Guangming Cao, Steve Clarke and Brian Lehaney

I have just included titles and authors, not direct download links to the PDF as unsure about copyrights. Some of my copies of the PDF are stamped with the date-time i downloaded. My email is in my profile if you want more.