Without a doubt it is, for me, "Pat & Hen": "Computer Organization and Design" by Patterson and Hennessy. I was mesmerised by this book as an undergrad and spent night after night devouring every page. It is a true eye-opener, explaining why computers are the way they are from the compiler down to the CPU design. You even get to design a CPU from scratch, i.e., from basic logic gates (in the older editions, at any rate). Magnificent and enlightening, a must read for anyone who really wants to know "what's going on under the hood".