Introduction to the Modeling and Analysis of Complex Systems
textbooks.opensuny.org
textbooks.opensuny.org
It has a much broader perspective than Allen Downey's "Think Complexity" [1], which is discrete systems (computational systems) focussed. Maybe you could think about Downey's book is a non mathematical approach to algorithms.
Scott E Page's book "The Model Thinker" is more mathematical, at an undergrad engineering level, and is more of a survey of models than a taxonomic overview of systems modeling, which is what Sayama's [2] book, above, offers. A thesis of Page's book is that there can be many ways to model a problem, and multiple models help.
[0] https://github.com/hsayama/PyCX [1] http://www.allendowney.com/wp/books/ [2] http://bingweb.binghamton.edu/~sayama/
Also, what's the best not-necessarily-open textbook on the subject? I've been meaning to dig into the topic in depth for a while, I could use a good textbook.