My own guess, based on watching my sales profile over the years, is that print, eBook and audiobook do not inherently cannibalize each others’ sales — it seems to me that for each there is a class of reader that is “native” to each — that is, there is a group of readers who strongly prefers print over eBook or audio, another group who prefers eBook strongly to the other formats, and a third group (correlated, I imagine, with people who have long commutes) who strongly prefer audiobook. I don’t think I lose a print sale by selling in eBook, or an eBook sale by selling in audio — rather, that selling in each of these formats is allowing me to expand my overall audience. Once again, this is an argument for remaining actively involved in all of the formats rather than throwing one (or more) overboard and putting all my chips on a single format.
John Scalzi breaks down the sales of his last novel, Redshirts, across formats now that Tor has moved the title from hardcover to paperback in print form, and it’s the kind of thing that’s fascinating for someone like me, who’s unnaturally geeky about this kind of thing. The part about digital and audio and print not cannibalizing the other format’s sales is of particular interest to me, because I’m beginning to suspect that the negative sales velocity that digital brought to analog music and movies just doesn’t exist for either books or comics, perhaps because the latter two are more active pastimes and therefore have more engaged audiences with more specific interests and habits surrounding their preferred format.