Hennessy and Patterson's Computer Architecture: A Quantitative Approach is the definitive textbook. They also have a 2nd textbook Computer Organization and Design. The former is more focused on people who might go after a Computer Engineering degree while the latter is more an exploration of microprocessor architecture relevant to a wide audience of programmers.

Previous editions are available free online and just as good for learning the big picture imo. Actually the older editions might be better for learning microarchitecture specifically because the more recent editions have cut some material on that in order to cover mobile and cloud computing.