I've read a lot of math books: I majored in math in college, got a PhD in CS (in cryptography, specifically) and I love doing math recreationally.  This is simply the best math text I've seen: it's got real math in it (quite difficult material in places, in fact), and yet has a lighthearted and fun-spirited approach.  The graffiti can be lame sometimes, but add something to the book.  And the exercises are all thoroughly answered within the text (as Knuth tends to do in all his books).<br><br>I have used some of the techniques in my research, or as stepping stones to more advanced study that then fed into my research.<br><br>I am a CS professor now, and although I would LOVE to use this text in a course, it's unfortunately too advanced for our students in a discrete math class.  Still, it's a gem on my shelves and a gift to the world.  Highly recommended.