Why should I advertise my print book if I mostly sell eBooks?
Customers often search and browse on Amazon without regard to book format. Amazon trained this agnostic search because a book’s landing page offers every format variety available. Of course, you will see exceptions to this rule in your customer search term report, where some customers include “Kindle” in their search terms. But more often, it’s the book title or the word “book”, both of which do not offer insight into customer intent.
Amazon ads has placements that favor one format or another. We should always expect this to change based on internal business priorities or new learnings from massive customer data. So, my point of view is this: get all the visibility you can by adding both to your ad campaigns.
On top of this, remember that in July 2020, the attribution model changed such that the ads dashboard only reports on advertised ASIN. This means, if you only advertise eBook and someone buys a print book from your ad (remember, because they can on the detail page), it needs to be in the campaign to be included in sales reporting. Otherwise, you won’t know that the ad is converting into sales on that format.