books.google.com - A stimulating excursion into pure mathematics aimed at "the mathematically traumatized," but great fun for mathematical hobbyists and serious mathematicians as well. Requiring only high school algebra as mathematical background, the book leads the reader from simple graphs through planar graphs, Euler's...http://books.google.com/books/about/Introduction_to_Graph_Theory.html?id=8nYH5OYEW24C&q=least&utm_source=gb-gplus-shareIntroduction to Graph Theory