I don't see much of an issue if a person is buying a physical book and has the option to download the electronic version. The energy involved in downloading the book is minimal after all, particularly when you consider that the person would probably be using a device that they already have. In the long run, it may even reduce the energy used since it may encourage people who would otherwise stick to physical books to convert to electronic editions.

Now if it was the otherway around, I could see your point. Printing and shipping a book takes a considerable amount of energy.