Any good algorithm book is a good start, so for example D. Knuth's work (he has written many books on algorithms, so pick the one with graphs in it) or Sedgewick's work (also many books, I use an older copy of Algorithms in C, though there are more modern ones).
You can also start with articles on wikipedia though, and go from there. Graph theory is very deep and mathematical on one side and more practical on the CS side. So if you for example are more interested in things like topological sorting, shortest path etc., you should read the graph articles on wikipedia which are related to computer science, but if you want to know more about the theory behind hamilton cycles etc., stay on the mathematical side .