After studying Japanese for quite a while and having read many books (maybe over 50-60 over a bunch of different genres), despite the fact I generally have tried to learn the readings of most words I come across, I always find myself discovering new words everyday whose pronunciation I don't know. Part of the time I can guess the reading since I know the common readings of the Kanji involved, but often not.
Nevertheless, even without understanding every reading and looking up every word, I still can understand things enough to follow the story.
My question is: Do most adult Japanese natives who read books really know each and every reading of every word they come across, or is there frequently times when they aren't sure? I'm sure some academic/intellectual type people who read very often might know nearly all of the readings, but I'm more curious about the average person who reads books off and on.
Part of the mystery to me is how one would learn all these readings. Most adult books don't have that much Furigana, and many of the more advanced words I am talking about wouldn't be heard frequently, if ever, in daily life. I just have a hard time believing adult readers would frequently look up Kanji readings, and instead would just aim to read such that they understand the overall flow of what is going on.
One example of one such books with a great many difficult words (for me) is this book, which is what I would call hardcore fantasy: https://www.amazon.co.jp/図書館の魔女-上-高田-大介/dp/4062182025
EDIT: As there was some discussion of the above-referenced book with difficult Japanese, I'll give a link to a review I wrote for it here.