Sterman's is the best all-round book on systems thinking I've read. Not too hand-wavy, not swimming in calculus, plenty of interesting case studies and practical advice that show deep experience. It's in a sweet spot for me.

Edit: as a note, it always seems to be substantially cheaper on Book Depository than on Amazon. I suspect that bots are, hilariously, in a feedback loop bidding up a book about feedback loops.