A good reference to the Gauss result on the structure of the multiplicative group of a residue ring
This is proved in detail in Chapter 4 (a short one) of "A classical introduction to modern number theory" by Ireland and Rosen. Specifically see Theorems 2 and 2' in page 43 (second edition) and their proofs.
Another reference is Proposition 2.1.24, page 22, in Henri Cohen's "Number Theory – Volume I: Tools and Diophantine Equation".