Just to add a few more resources – ye olde digital design books are really fun to learn how (sequenced) microcoding was seen as one of the highest-level tools in the toolkit of digital design in general up to the early/mid 80s:

– "Computation Structures", an old MIT course book, which uses a pedagogical processor similar to a microcoded PDP-11 or a 68k, to illustrate microcoding as the highest level of digital design – the processor they implement used a two-level microcoded processor to implement two types of processor on the same microcode interpreter engine, complete with built-in monitor/debugger.

– "The Art of Digital Design", an older course text, which culminates in the implementation of a PDP-8 clone;

Finally, if it helps look up more literature, another name for microprogramming in the literature is "interpretive microcoding" – one that makes the distinction between ye olde microprogramming and modern "microoperations" pretty clear.

It also clarifies why two-level micro-coding might exist, especially in the microarchitecture of the original 68k: you write in high-level microcode an engine interpreting a target ISA; this high-level microcode interpreter is then interpreted by low-level microcode that directly activates control lines. Two levels of interpretation seems baroque, but somehow it was fast enough for 68k workstations.