There are a number of good books published that provide examples of network graphs. Here are a few to get you started:
In addition to his previously mentioned website, Manuel Lima has produced a book with the same Visual Complexity title. In this volume, Lima features some of the very best examples of network visualizations, and categorizes them into specific types. The book can be found at http://www.amazon.com/exec/obidos/ASIN/1568989369/visualcompl0f-20/.
Taschen is a German publishing house that produces a wide array of specialty books, often available in large formats. Information Graphics (http://www.taschen.com/pages/en/catalogue/design/all/04984/facts.information_graphics.htm) provides a stunning history of exceptional graphic displays, including many of the network variety.
Alberto Cairo is a graphic designer and educator who published a book titled The Functional Art that examines the process behind creating compelling visualizations, including those involving networks at http://www.thefunctionalart.com/.
Connected is an influential book written by Nicholas Christakis and James Fowler that focuses on how our connections influence our own lives and those of others. (http://connectedthebook.com/)
The BarabisiLab site also plays host to Linked, a book authored by Albert-Laszlo Barabasi. This book focuses on the interconnectedness of all things in our society and how they interrelate and influence our lives.
There are also many books that deal with the more technical aspects of graph theory and creation. A simple web search will yield numerous results if you wish to understand the mathematics behind your graphs.