The characterization of norm compactness in $L^p$: you want the Frechét-Kolmogorov theorem, usually stated for $X=\mathbb{R}^n$ and $Y:=\mathbb{R}$. The proof follows quite easily formfrom Ascoli-Arzelà by mollification. You can find it on HaïmHaïm Brezis' Functional Analysis.