This is a reading series I like a lot, some people may find it boring. I found the wide variety of subjects covered e.g. fair trade, cultural differences, even an article on Murphy’s Law useful to expand my scope of reading in Korean, and of course, there are lots of articles on Korean culture, e.g. 5 day market, traditional stories. When I was reading the Korean culture articles, I did not feel their significance, until some time later one day when I was visiting Korea/watching a drama, then suddenly I would recall some of the stuff from one of them and found they helped me understand a bit more on Korea.
This is a chapter in book 5, you can read it to judge if this book is appropriate to your level. I have only finished book 4 and 5 (currently reading book 6), in my opinion there are a lot more difficult then the corresponding TOPIK level, if you have no difficulty in understanding book 4, then you probably have no difficulty in getting TOPIK 5 and so on. However, this book has relatively few explanations for the meanings of the words, (or maybe my vocabulary is too bad), I found I need to check the dictionary very frequently.
After every chapter, there are vocabulary, comprehension and writing exercises.
Content for book 4, 5 ,6 for your reference.
For you to see the difficulty, this is from book 4
This is from book 6