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.