I agree that things can overlap. For example, I read a Doctorow book in ebook and then bought the paper version for someone else as a gift.
I guess when I refer to what the publishers call 'cannibalizing' I don't mean art books, cookbooks and the like. I mean (and I think they mean too) that in the past you would have bought the hardback of that particular book, and you are buying the ebook instead of this hardback you otherwise would have bought. So, if there is no snowballs's chance you ever would have bought the hardback, it is not a cannibalized sale. And likewise, if there is no way you would have bought the ebook, great, enjoy your art books and cookbooks and whatever
I just wonder for something that hasn't got pictures and is just plain mass-market words, how serious the 'problem' of 'I would have spent $40 on the hardback and now I am not' really is. It seems like a lot of us get our books---e and otherwise---used, on sale, in remainder bins etc. I just wonder how many people truly are in the position where they actually used to buy full-price hardbacks and are now buying the ebook version instead.