Nonlinear Dynamics and Chaos by Steven Strogatz is pretty good. Nonlinear dynamics is one of the backbones of complex systems, so it's worth delving into. Graph theory/topology is a close second. Complex Networks by Latora, Nicosia, and Russo is a nice introduction to the graph-theoretical perspective. Complexity: A Guided Tour by Melanie Mitchell is a nice and brief introduction. I can also recommend Spin Glasses and Complexity by Stein and Newman. Spin glasses are basically the "model organism" of complexity science, so it wouldn't hurt to get acquainted with them.

Thank you!