Here are some grammar and drill books. I believe it's always a good idea to have at least one book of this kind.
I skimmed through parts of the two 다락원 books. They looked like well organized books to me.
* Korean Made Easy - Basic/Intermediate/Advanced/Vocabulary, published by Darakwon(다락원)
* Korean Grammar in Use - Basic/Intermediate/Advanced, published by Darakwon(다락원)
* Korean language series books and online courses from major universities. They have a good reputation.
- Seoul National University (서울대학교)
- Yonsei University (연세대학교)
- Sogang Unversityi (서강대학교).