I've been reading Inside the Machine by Jon Stokes - An Illustrated Introduction to Microprocessors and Computer Architecture. It looks at Intel, IBM Power PC, and Motorola chips (i.e the ones used in PC's and Mac's), including the latest Core 2 Duo chips.
I'm not much of a hardware person these days, but it was pretty interesting to read about the techniques used to push performance. Apart from bigger caches, I had wondered how modern CPU's used the 100's of millions of transistors they now contain. (A Core 2 Duo has 291 million transistors as compared to the original Pentium's 3 million.)
Considering the subject, the book is well written and easy to read. If you're at all interested in this area, I'd recommend it.