I was looking for a reference which discusses the structure of finite integral extensions of $\mathbb{Z}/p^k\mathbb{Z}$. In particular, I am interested in understanding what the abelian group of its units looks like and is there some Galois/Finite field-like theory for them.

I tried looking for some but can't quite find things directly related to this problem.

Edit: I should mention I can prove things for degree 2 extensions. But for higher degree extensions I can't quite find a complete picture.