Although it's an "introduction", this gem of a book ends up in some quite deep territory. Trudeau starts off with some basic definitions of set theory concepts and then moves forward to define graphs in those terms.
Concepts such as planarity, connectedness, polygonality and colourings are quickly and smoothly reached, and the back end of the book covers genuses (which I thought was pretty incongruous for an "introduction"). Proofs of the Five Colour Theorem and the Heawood Colouring Theorem are included, as well as demonstrations of Euler's Formulae and Kuratowski's Theorem.
Trudeau's style is completely non-indimidating and patient - almost conversational - and he conveys a real enjoyment of the subject. Non-mathematicians will be able to follow almost all of his arguments quite easily and, for this reason above all others, he deserves 5 stars.
P.S. I spotted quite a large howler towards the end of the book: the Four Colour Theorem is stated as having "just been proved" - it was proven in 1977, which goes to show how old this book is!