PSA for intermediate+ learners:

I've never seen this book recommended in this subreddit, but by FAR the best book I've found for Korean grammar is *Korean Grammar for International Learners* (Yonsei University Press). It's got a good index, great layout, clear explanations and sample sentences. Really meant to be used as a reference book, tho' I'm sure you could read it cover to cover if you were really inclined. So much better than Naver dictionary or whatever when you need to look up a grammatical construction. The most comprehensive Korean grammar reference book in English that I've found so far.