What are copyright issues regarding textbook use?

I'm in the process of starting my own teaching business. And my question is about text books. I would like to know: do I have to have a permission to teach from any of the textbooks at my center? In other words, if I tell my students to buy authentic copies of certain textbooks, e.g. academic reading encounters, and then teach the book at my center, is that legally ok? Or I have to get a permission from Cambridge to do so? One final question: can I just buy one copy of a listening textbook and use it with my students, instead of every one of them buying a copy. I just buy one authentic copy for the instructor who can play the tape and then ask the questions found in the textbook and discuss the topic with the students. I want to know is it legal to do so: buy one copy for the instructor, from which he/she can read the questions or play the tape? Thank you.