If I have a clause in my contract where any "Derivative" works are owned by the company -- what are the limits of this were I to create something new?

Specifically, I'm trying to understand what happens in the following example. Say Author Z creates a copyrighted Book Z and develops a coaching program + methodology around Program Z. I then become "certified" to teach Program Z and sign a contract whereby "any derivative works are the sole property of Author Z" How far to the tentacles of this agreement reach? For example, were I to write my own Book ABCXYZ that incorporates fair use material from Book Z (maybe 1 chapter on the general subject) while also adding a substantial amount of new material above and beyond the Z material alone, does this trigger the derivative clause and grant Author Z full rights to my entire Book XYZ? Or does the "derivative" clause only grant permission to the portion of my book that uses the Z material specifically?