Next/Previous Post Navigation within Parent Category

I thought I could use previous_post_link and next_post_link on my single.php page but I realized I can’t. I checked the Codex and found out I can exclude a category but not add one…

Here is what I want(ed) to do :

Books (Parent)
– Paper (Child)
– eBook (Child)

When I’m on a page in the eBook category, I want to be able to navigate to the next and previous post in the Books category, not only eBook. But I can’t. I can only navigate within the eBook, child category.

Is there a way to specify the category I want to navigate? Therefore the Parent category? Codex says I can exclude a category but can’t add one. Weird.

Thanks for your help!