Reddit reviews The Routledge Intermediate Korean Reader (Routledge Modern Language Readers)

Routledge Intermediate Korean Reader

Advanced Korean

KLEAR Korean Reader for Chinese Characters is also a graded reader but it's based on progressively harder hanja, not Korean grammar

I was at a bit more advanced level when I first used this reader, but I can highly recommend it when you feel up for it. I suggest you look at the preview and decide what you think of the first story (the difficulty ramps up a bit on the later sections of the book).

At this point, I've gone through that text countless times now, with a focus on improving my reading speed and comprehension, and it's really helped build up my confidence to tackle native materials.

I've moved on to trying to read webnovels and webtoons on kakao and I try to use a routine I worked out from going through that book:

1. first pass I just do my best to read through the text without any dictionary
   
2. transcribe the text into a word document
   
3. go through everything with the help of google and naver dict and try to leave nothing unknown
   
4. re-read it again a few times and aim to get faster
   
5. schedule myself to come back to re-read the text on another day
   
6. repeat step 3 on anything I forgot. I might do some more re-reads if I'm still not comfortable
   
   I guess TLDR: re-read a lot. That's just what's worked out best for me so far. Your experience may vary.
   
   That same company seems to have a more beginner level reader text but I've never used it so I can't give an opinion on it. It looks like it also has a preview of the first story.

They are popular among Mandarin learners. Here's an example, basically built on the HSK lists. While the HSK lists are not precisely frequency based, they do basically introduce words based on how commonly encountered they will be in writing and also how complex.

This could be a parallel in Korean.