These seven Korean E-books (PDF + audio) can be useful for those students who prefer online resources:

1. TOPIK I Vocabulary in 40 days + TOPIK II Vocabulary in 50 days: https://goo.gl/TXZ7mi 

2. Sogang Korean student textbooks: https://goo.gl/iFB6o4 

3. Yonsei Korean: https://goo.gl/wc7cRm 

4. TOPIK Esential Grammar 150 PDF textbook: https://goo.gl/1pYQMK 

5. Customized Korean textbook: https://goo.gl/oDHnua 

6. Korean for Overseas Koreans: https://goo.gl/8PtKpQ 

7. Korean Immigration and Integration Program: https://goo.gl/RfMV9B