It ultimately depends on the level of abstraction ("emulating a CPU" is ambiguous): emulating the logical gates of a CPU is an entirely different matter from em

It ultimately depends on the level of abstraction ("emulating a CPU" is ambiguous): emulating the logical gates of a CPU is an entirely different matter from emulating the instruction set; besides, generally one emulates an entire system, not just the CPU. If you're looking for the former, the canonical course is "NAND2Tetris". If you take the class on Coursera, the first (of two parts) covers the hardware. I've also seen two very interesting books, but I haven't read them - "The ZX Spectrum Ula: How to Design a Microcomputer", and "Designing Video Game Hardware in Verilog". Courses at this level are generally exactly defined. In the second case - emulating at the ISA level, the canonical entry is to emulate a CHIP-8, since it's simple and relatively well defined. At this level, targets are rarely exactly defined. CHIP-8 for example is well-defined in theory, however, for example, looking at the Wikipedia page won't suffice, because there are many extensions. Additionally, CHIP-8 has no standard clock speed. But it's easy to find the resources, and you can end with an exactly working system (the same can't be easily say, for example, of a NES or a C64 - you'll find conflicting information).