Here’s a thread on Twitter with systems design / systems thinking resources:
For convenience, here are some of the links from the thread (auto-extracted):
If Russ Ackoff had given a TED Talk… - YouTube
Systems Innovation - YouTube
Systems Innovation Platform
Systemic Design - Theory, Methods, and Practice | Peter Jones | Springer
Thinking in Systems: A Primer by Donella H. Meadows
The Fifth Discipline: The Art & Practice of The Learning Organization by Peter M. Senge
Buy Thinking in Systems: A Primer Book Online at Low Prices in India | Thinking in Systems: A Primer Reviews & Ratings - Amazon.in
Systems Practice: How to Act in a Climate Change World: How to Act in a Climate-Change World: Amazon.co.uk: Ison, Ray: 9781849961240: Books
The Systems Thinker – Authors - The Systems Thinker
Publications « Systemic Design Association
System Dynamics Self Study | Sloan School of Management | MIT OpenCourseWare
Systemic design toolkit – Namahn
Mental Models: The Best Way to Make Intelligent Decisions (109 Models Explained)
Jay Wright Forrester - Wikipedia
Futures wheel - Wikipedia
and the text from many/most of the replies (n.b., there’s at least one useful photo you should visit twitter for):
“What would you consider to be the best content to learn systems design? (not design systems, but systems thinking)”
“The Systems Thinker is another gold!”
“Thinking in Systems: A Primer by Donella Meadows https://t.co/NLhEMYspHu via @amazonIN”
“@killerswan Fiction novel writing, biology and medicine history of discovery and errors, history of beginnings of airplanes/ space flight, people’s oriented history of events books, meteorology and weather prediction”
“Reading Philosophy, specially thinkers that analyze structures and systems I.e Hegel, Marx, Deleuze, Foucault”
“I’m working through Ray Ison’s Systems Practice (part of a Masters ) https://t.co/vBdX8DdoWD . Has the concepts that everyone else in the thread has mentioned.”
“I’m listening to Team of Teams - this morning I made a note to read up more deliberately on #systemsthinking - very timely, thank you 
”
“Donella meadows dancing with systems.”
“End of Average, Todd Rose is a great primer”
“Can’t see a mention of Patricia Shaw in here anywhere & she’s really interesting - takes stuff in a different direction. Also found Werner Ulrich useful when it comes to using systems thinking inside hierarchical organisations.”
“Was Forrester mentioned? https://t.co/Jkqz4qDmHV”
“Lots of great suggestions! Also really important to consider consequences over time. I really like the futures wheel.\nhttps://t.co/EkOEYq3C7m”
“Any content on category theory”
“Take a look at the explanation of a “paradigm shift” in the novel Ham on Rye … anyone who reads that will always have a greater understanding of systems thinking”
“https://t.co/31uK7B4sOR is a gold mine, look for “systems” under mental models.”
“1/2 Idealized Design by Ackoff for this particular question. I highly recommend NOT to start with 5th Discipline as it conflate broader concept of systmes thinking with system dynamics.”
“Not just content but also a great community - https://t.co/QPyGfqOQVB with local chapters, organizing events, wokrshops and providing support to new system thinkers 
”
“+1 Meadows \nI also find Systems Innovation on YouTube useful: https://t.co/QZNbtO9QUr\nAnd @Sys_innovation is worth a follow”
““System Design” and “Systems Thinking” are not the same thing. But you can apply Systems Thinking to system design. I can recommend “Systems Thinkers” (OU) as a good intro. I would then first read “General System Theory” before diving into the different schools of thought.”
“I was introduced to the subject by Meadows’ “Thinking in Systems” and I loved it.\n\nLater I realized that Factorio (video game) is a great way to represent a system in motion.”
“Biology”
“A lot has been shared here. Also check this and follow the links https://t.co/WtSvzJEhZy”
“Thinking in Systems is the book that got me into system thinking. I have written a couple of Medium articles of this and UX, I’d be interested to find out what people think. https://t.co/PUb5XBhYbb”
“this book defines product innovation as a complex adaptive system and introduces a framework for collaborative system thinking in the product field 
\n\nhttps://t.co/VoKpAlm3eP”
“Would recommend this free course as well\nhttps://t.co/Y67FQnRacZ”
“Semi related … \n\nScale by West\nhttps://t.co/6Lp1WCxZaM\n\nBook of Why\nhttps://t.co/34AUCiou1n\n\nHidden life of trees \nhttps://t.co/0sGp8WD4yv”
“The Fifth Discipline, by Peter Senge https://t.co/DzAYdKFGS8”
“@ruthmalan Systems Architecting - Creating & Building Complex Systems by Eberhardt Rechtin, 1991”
“Besides the usual:\n\n- The Systems View of Life (Fritjof Capra)\n- Strategic Modelling and Business Dynamics (John D. W. Morecroft)”
“No Ackoff yet… Start at minute 1:30; just 10 minutes… \nhttps://t.co/wknz8wpZle”
“@ruthmalan I’m fairly new to this, but I’ve learned a lot from these books:\n- Thinking in Systems (Meadows)\n- The Fifth Discipline (Senge)\n- Design Unbound (Pendleton-Jullian & Brown)\n- Drift into Failure (Dekker)\n\nAlso highly recommend “How Complex Systems Fail” https://t.co/rYY25VBHgx”
“Visual vocabulary of systems: https://t.co/afzoy08E7N”
“I teach systems for interaction designers at CCA. I use chapters from all these books and my students read all of the Donella Meadows Thinking In Systems. There are tons of resources online from the Systems Thinker and Systems And Us sites. https://t.co/Xx7f68RQX3”
“@ruthmalan “Dynamics in Action” by Alicia Juarrero is a great book on how minds can be understood as complex adaptive systems. It’s not about design, but it’s full of insight into complex systems. https://t.co/RnJdEsTHue”
“Hugh Dubberly’s blog has a lot of good content and links out to plenty of resources. https://t.co/yz5wH4ti0k”
“@ruthmalan I would also suggest Pegasus Communications books like Study Notes in SD, @tomfid’s blog, MIT’s course https://t.co/MJZlh7kMnY and Gene Bellinger’s resources https://t.co/iCgyOjHdON”
“@ruthmalan Please consider the work of @NoraBateson, #Symmathesy, #WarmData, putting the context back into Systems Thinking.\nIt’s not all boxes and arrows, after all.\nI hope that helps, it took me some years to begin to understand.”
“I approach systems thinking through storytelling. Shows like The Wire (criminal justice system), Borgen (Danish politics system), and The Expanse (fictional future technological/political systems) are some good places to start.”
“+1 on Meadows’s “Systems Thinking” primer. For a more actionable book, see “Growing Wings on the Way” by Rosalind Armson. This PDF is also useful: https://t.co/c9exyk5QyZ”
“@ruthmalan To date I’ve preferred the older “book” units…as they conveniently integrate with the existing “shelf” systems I already have in place ;)”
“I’d recommend a look at this book edited by @redesign et al: \n\nhttps://t.co/nGltNVhlTr\n\nAlso, see the proceedings of the RSD conferences.\nhttps://t.co/5AnAh8szOH\n\nLastly, everything by @josinavink and @ManuelaAguirreU is ace.”
“Have you checked out the Systemic Design community & events? https://t.co/PmJu3aP40F Folks like @DrAlexRyan been working at that intersection for a long time. Also, @snowded and Cynefin, @cyetain & whatever he’s riffing today, @Terry_Irwin @camerontw and work on transition design”
“https://t.co/MtaDJD1cLR”
“Donella H. Meadows, Leverage Points: Places to Intervene in a System (1997)\n\nJodi Forlizzi and Katja Battarbee, Understanding Experience in Interactive Systems (2004)”
“@jarango might have ideas here”
“And @boonych will have some good tips”
“This is interesting and useful too: https://t.co/DCiXVU6sE3”
“Watching eagerly for different recommendations than the usual. \n\nUsual suspects include ‘Thinking in systems’ \nhttps://t.co/CIO5V8OFGS or ‘the Fifth Discipline’ https://t.co/d2rV668TSY”
“Really recommend this book. https://t.co/ktWQXz7JSe”