"Computer Architecture: A Quantitative Approach" is the standard text. I like "Modern Processor Design: Fundamentals of Superscalar Processors" for an explanation of how superscalar processors work.
If you can find a copy, "Computer Organization" by V. Carl Hamacher, Zvonko G. Vranesic and Safwat G. Zaky, published by Mcgraw-Hill has, at least in my copy of the 2nd edition, a nice chapter on microcode control units.
If you can find a copy, reading the prior chapter on hardwired control units might be reasonable as it is somewhat "background" for following the microcode chapter.