ld's favorite English course. Interchange is a four-level, American English course that has been used by over 50 million students worldwide. This Intro Student's Book with eBook includes access to an eBook with audio. It has 16 units, four progress checks, and a Grammar Plus section to ensure students receive all the practice they need. Inside each book is a single-use code for the eBook with audio, which is mobile-friendly and can be used on a smartphone, tablet, or computer.