For textbooks, I'd definitely recommend the Integrated Korean series by KLEAR. I have Korean textbooks made and used by Korean universities (i.e. Yonsei's 한국어 series and Seoul National University's 한국어 series), but KLEAR's Integrated Korean has, in my opinion, richer grammar explanations which are more effective if you're self-studying. If you're looking for grammar books to complement what you've learned, I suggest either Korean Grammar in Use (it has both Beginning and Intermediate grammar books) and Korean Grammar for International Learners. :D