I’m not sure if I have any great answers. Certainly doing it a lot helps, and in this respect I think books and manga are different. In Manga you have a lot of context clues and it’s mostly conversation, while most books give you nothing but the text. (Obviously non-fiction books can be either here). So you need to work on making sure you understand text as text.
One thing I did is read a lot of short stories in places like Aozora Bunko. That way, even if the stories were hard, they weren’t super long, so I could read them relatively quickly. As opposed to reading something like 沈黙 (Silence), which took me a longggg time to finish.
On a simple technical level. I would say to err on the side of looking up words you don’t know in the beginning of the book, as they will likely continue to appear. For instance, to stick with the example above, due to the theme of the book, there are a lot of words like 司祭、神父、背教、踏み絵 etc that I was very unlikely to know, but were in the book a lot. So reading the book got gradually easier over time.
Don’t be afraid to re-read things if you need to. But also try not to get stuck on one sentence forever. At some point you may need to understand that you just don’t get it and move on. Sometimes, something later in the book will make you able to understand a previous sentence, or realize you did in-fact understand something wrong. Like thinking a character said no to something they actually said yes to.
So I don’t know if this is advice, but these are my main suggestions. I realized I wrote mostly about fiction here, but I think much of the same is true for non-fiction. Also, one personal suggestion is to read a variety of topics and types of books. That will give you the most bang for your buck. I think if you approach it from a “reading books” perspective and not a “learn Japanese” perspective, it will also be better.
(unless I’m reading an ebook where I still do it every now and then just because it’s pretty fast anyway and being able to understand what’s happening from context doesn’t mean I actually know 100% of the words all the time)
