To learn more about Korean hip-hop in English, Song Myoungsun’s Han-guk Hip-hop is good.
I usually recommend people read journal articles on K-pop instead of books because of the lag time to publication, since books get out of date pretty quickly. But there are so many excellent books that are on topics other than K-pop that would be beneficial for K-pop fans as well. I have a pinned tweet on Twitter where I recommend a ton of books on Korea. Reading a short historical overview of Korea such as Hwang Kyung-moon’s A History of Korea could help many fans. Reading a blog like James Turnbull’s Grand Narrative will help fans understand gender issues in Korea, and the blog often intersects with media cultures. There are also podcasts often made by people who have been following K-pop for a decade or longer that can be quite informative, if you’re a regular podcast listener. On Twitter, I hope everyone follows @AskAKorean because that’s one of the most informative accounts. Ask A Korean is also a website.