Inspired (or shameless ripping off) another thread (https://www.mumsnet.com/talk/what_were_reading/5568805-which-books-have-you-decided-not-to-finish-and-why?page=1) I started to think about books that I hated when I started reading them, decided to persist for a while and was so glad not to have given up.
For me, the book that best falls into this category is Vernon God Little by DBC Pierre. It has a first person narrator and the protagonist is a teenage boy, growing up in poverty in a small town in the deep South of the US whose friend commits a school shooting.
I struggled so much at first because it's written in dialect. I couldn't get into it because of the language and almost gave up. I decided to give it a couple more chapters and was so glad that I kept reading it because once I'd got into the dialect, I absolutely loved it.