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 Level A Student's Book with eBook includes access to an eBook w/ 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.