The Art of Electronics - differences between book versions?
International editions are substantially cheaper, in both meanings of the term. They are printed on lower-quality paper, with smaller print, and black-and-write ink. (But AoE has no color sections anyway.)  

Hardcover and paperback versions refers to the binding.  I don't think there are any differences in content.

Before buying the book, take note that a 3rd edition will be published Real Soon Now: http://www.eevblog.com/forum/chat/the-art-of-electronics-3rd-edition-finished-writing-and-copy-editing/

[Editor's note: The third edition is now available]