Discourse won’t let me post in Korean Beta - Feedback from Korean Users because I’ve posted 3 times in a row… 
Finished the 두루책방 - Free (series) | L0-20 series, and high teens seems about right for the most difficult books. A handful are at L20, but I’m pretty sure they don’t have any TOPIK 4 grammar, and they don’t include idioms or other things that might make the language difficult. Not a big deal, I’ll leave it up to you whether to bump them down a level. 
Submitted feedback on this a while ago, but book 1, 달의 그림자 그림자의 바다 - 예스24, is missing from 십이국기 (series) | L35??. The current book 1, 마성의 아이 | L35?? should be book 0, as it’s a prequel.
And just gonna leave this here… 
