Molecular Biology of the Cell is a wonderful book (my first PhD advisor was one of the authors of the 3rd edition), but if you're more interested in the purely

Molecular Biology of the Cell is a wonderful book (my first PhD advisor was one of the authors of the 3rd edition), but if you're more interested in the purely genetics side of things then I'd recommend Genes by Lewin - it's definitely the book I relied on most during my undergrad degree, and it's written in a way that largely lets you bootstrap from not having a strong biology background.