There many books on Graph Theory. The online resources on Graph Theory, however, are not satisfactory. Here’s few introductory resources I looked at (ignoring the videos on the YouTube)
https://www.cl.cam.ac.uk/teaching/1011/PrincComm/slides/graph_theory_1-11.pdf
https://economics.mit.edu/files/4620
I also recommend Josh Reuben’s tutorials:
A Short Deep Learning Study Guide
For some of you the MathWorld would be more appropriate: Graph . You can follow the links there and get deeper into the Graph Theory.
Let’s not ignore the Wikipedia article on Graph Theory
My new favorite Scholarpedia has many articles related to Graph Theory as well:
Scholarpedia: Computational Neuroscience articles
Suresh Emre
Renaissance Universal 