Do students need traditional books?
No, Students don't need a book. They DO need good resources. 

Maybe its an eBook or a PDF compiled from your teaching notes or outline/slides (Hopefully cited well) and things like short stories needed for a language class can often be found online in their entirity, for free legally, when copyright has expired. 

For a language class, using a Sherlock Holmes story could prevent students from needing to buy a book as they're out of copyright. You can also look for material that is licensed as Creative Commons which allows for free use, such as material from MIT open courseware, Khan Academy, and P2P Univerity. You can learn more about Creative Commons for Education here.

Long story short, make good content, and use it legally, either by using stuff people have given to the public, or under doctrines such as Fair Use.