References to learn more about graph laplacian.
Suggest these free available resources. Most textbooks on graph theory only glance at "free probability theory". 

Lectures on the Combinatorics of Free Probability (London Mathematical Society Lecture Note Series): 
http://www.amazon.com/Lectures-Combinatorics-Probability-Mathematical-Society/dp/0521858526

Speicher lectures: http://www.mast.queensu.ca/~speicher/papers/lectures-IHP.pdf