When we were in college, things often happened to English teachers at home, so we often missed classes and taught by ourselves. In order to make us study better, the teacher gave us two books, New Concept English 3 and New Concept English 4. I think these two books are much easier than the textbooks issued by the school, and they are especially suitable for self-study, because they have no difficult words and can be learned well without the help of teachers.