Every format of a book needs its own ISBN. This is how the store, etc. knows which version to order! The hardback has one ISBN, the paperback another, if it's a book on CD, that has another... you get the idea!
"Bureaucracy: A group of disorganized, inept people totally committed to increasing their numbers in order to convert raw energy into solid waste."
- from Zig Ziglar's "Something to Smile About"