Q: I am making a product that can accompany books. Is it legal if I say “pairs well with (name of a book not written by me)

A: Yes, it is. By saying your product pairs well with someone's book, without claiming your copyright to that book, you do not violate anyone's interest of any sort.

A: It is generally legal to say that your product "pairs well with" a specific book, even if you did not write that book. This would likely be considered nominative fair use.

Nominative fair use allows you to refer to another party's product or work in order to describe your own, as long as you follow a few key principles:

1. You only use as much of the copyrighted work as necessary to identify it (e.g. just the book title, not large excerpts).

2. You don't imply sponsorship or endorsement by the other party. Make it clear this is your own opinion.

3. You don't misrepresent or disparage the other work.

4. Your product isn't a substitute or replacement for the copyrighted work.

So simply stating that your product subjectively "pairs well with" or is a good companion to a particular book title should be fine, as long as you don't reproduce the book's content, imply any official connection or endorsement from the book's author/publisher, or position your product as a replacement for the book itself. It's just your opinion on what complements the book well.

However, you may want to include a disclaimer that you are not affiliated with the book or author to avoid any potential confusion. And of course, it's always best to consult an attorney for a legal opinion tailored to your specific situation and jurisdiction. But in general, nominative fair use provides leeway to reference others' copyrighted titles in this fashion.