Although I have often wondered how fans of fiction can collectively know the pronunciation from a name unique to a book. I've rarely ever heard someone say the name of a character I have only read and thought "huh, that's different than what I thought it would be."
There are some old interviews with George RR Martin where people ask him about various characters, and GRRM would adjust his pronounciation to match the person asking the question. So he's pronouncing names differently in different interviews depending on how others pronounce them. I wonder if it is to make the other person comfortable, or if he just doesn't have a canon pronounciation.
I think it's because writers take care to make the pronunciation guessable form the spelling. English is infamous for it's very inconsistent writing rules, however there are "rules". More like heuristics, but usually it's possible to write a word in such a way that others can guess the pronunciation, unless that specific word already has an accepted official spelling that is different.