A coursebook is the book a class uses week after week for one subject — the book the lessons are built around, with units, tasks, and a path through the term. It is not every book on the reading list, and it is not the extra exercise book (that is a workbook). British teachers and publishers say coursebook a lot, especially in English class. American teachers, and most Taiwan classrooms, say textbook. Though actually, if you pick up an Oxford or Cambridge English book, the cover still says coursebook even when everyone in the room is calling it 課本. Write it as one word.