O'Reilly used to have a program in place where you could get the eBook copy of any of your physical books for a few dollars extra, even books published long before modern-eBook sales were thing.

I've seen some places that will bundle an eBook with a print copy at a discount.

More than a few books I've bought from Lulu.com had a note in the first few pages with a URL to download an eBook or PDF copy.

I've also emailed authors and asked them for an eBook copy of the book with proof of purchase.