O'Reilly and Microsoft Press have recently switched to using eBooks as the preferred media for distributing review copies. Like all book reviewers, I receive more books than I actually review. However, since I live in Hawaii, the physical and energy waste of a book that goes unread and for which its difficult to find a home (no colleges on my side of the island) is apparent.
I hope other publishers follow in their footsteps and switch to eBooks (preferably not non-reflowable PDF!) as the default format for reviewers.