John L. Hennessy, David A. Patterson — Computer Architecture: A Quantitative Approach — 4th ed., Morgan Kaufmann, , 704 pp. ISBN 0-12-370490-1
Computer Architecture: A Quantitative Approach is probably the most up-to-date and comprehensive introductory text for computer architecture, covering a broad spectrum of topics from micro-instructions to multi-core parallelism. This book is different—from the aging Advanced Computer Architecture: Parallelism, Scalability, Programmability by Kai Hwang (1992, now out of print) for example—in that it takes a quantitative approach, motivating most statements by hard numbers, simulations and benchmarks.
However, it is still an introductory level book and by its breadth necessarily presents some topics a bit too cursorily; to my opinion, there are some interesting proofs that are missing. The reader should therefore complement its reading by other, more specialized books.
Nevertheless, Computer Architecture: A Quantitative Approach is something of the definitive introductory reference. A must read.