I don't know what your level of knowledge is with CPUs and instructions, but if you don't know anything it is probably easier to look at 8-bit micro controllers first, especially since most can be considered RISC (reduced instruction set).
Also, "how an instruction works" is a bit vague, are you asking on a language level? Transistor level? Somewhere between the two? I mean, fundamentally an instruction is the series of 1s and 0s that tells a CPU what to do, the closest "language" is probably assembly.
I don't know of any good articles/websites but I can point you to some textbooks.