I must admit my preference (eBook versus physical cookbook) depends on the cookbook. I really like my eBook version of "The Flavor Bible" because its searchable. Want to find out what goes well with Worcestershire sauce, and for whatever reason, they don't have an entry for it, then I can search, find other items that have Worcestershire sauce listed under them, create a note, list the items, and voila, I have the information. It would be incredibly hard (read, impossible) to do this manually. I do love physical cookbooks, though, despite space and green considerations.
As for supporting the publishing industry, I'd be willing to bet publishers make MORE off digital cookbooks than physical ones. Not only are there no printing costs involved, they're not having to pay for wherehouse space to store them, bodies to package and ship them, shipment fees, and there are no remaindered books coming back that need to be disposed of. But there is an issue related to digital cookbooks many folks haven't thought of, that you might want to contemplate in making digital purchases. Hopefully this won't apply to you for many decades to come, but if you're hoping to leave your digital cookbook library to a child, you're out of luck. When you buy an ebook, you don't actually purchase a copy, you're licensing its use. For you alone, its non transferable. So your collection can't be left to your favorite library, a cooking club, or that daughter who loves to cook. It goes poof.
As for supporting the authors, I'm not sure how much they actually make off a cookbook. Its probably far less than we could imagine possible. Several years ago, a very prominent quilter and quilt book author, who probably made close to top tier as far as quilting book deals go, told me that quilting authors made about a buck a book. That would be about a buck out of a $25, $30 quilting book, a mere fraction of the cost. Many of them made their money off the book in other ways, as a promotional tool, using it to launch specific quilting classes, or buying it wholesale from the publisher and selling it to their students (at quilt shows and seminars) for full retail. I suspect the same may be likely true for most cookbook authors as well.
If you really want to support the authors (and that's laudable in my opinion), there might be other ways of doing it. Find the author's website, and buy their cookbook through their links. Odds are, they may be affiliate links, and the author may make more from their affiliate commission (up to 8 percent of the purchase price) than they would from the publisher. Let me give a couple of examples (and before anyone asks about it, I have no connection with any of these folks or their websites, no financial ties, they were chosen almost at random to provide examples for the OP). The other day I was searching for something, and stumbled upon this page from Michael Ruhlman's website http://ruhlman.com/2...need-your-help/ which interested me because it had pictures of several books, including a book on schmaltz he's releasing in October I'm very interested in. If you hover over those book images, those aren't traditional Amazon affiliate links, but they're worded in a very specific way, including with a /ruhlmancom on the end, that leads me to believe there's an affiliate relationship there. Which means, most likely, if you click on the image to go to Amazon, and purchase stuff in the next 24 hours, Mr. Ruhlman is probably making a commission for referring the purchase. Or if you go to Dorie Greenspan's site, http://www.doriegreenspan.com/ then click on "Books" she's got a whole Amazon astore set up there, with not only her books, but recommended cooking equipments. If you hover over the pictures or the text links, the links will say "doriegreenspa-20" somewhere in them. That's the affiliate code. So you're helping support the site, and presumably the author. Or since Lynn Rosetto Kasper was mentioned above, I found her website, http://www.splendidtable.org/books , went to the books section, and if you hover over the links, say for the book on top, that Amazon link with the "tsplent-20" at the end is an Amazon link, and I think the other two (B&N, etc.) links are also affiliate codes.
Whatever format you end up buying, new, used, digital, if you want to help your favorite cookbook authors without it costing you a dime, find their websites and look for affiliate links. Even if you buy something unrelated, golf clubs, a TV, video games, you name it, as long as you make the purchase within 24 hours of going through their affiliate links, they get a commission.
(And again, I have no affiliation whatsoever with the above, no financial interest, they were chosen as semi-random examples).