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?

This is a tricky question as you are entering a gray area if you are using material from Book Z to inspire your own book. Derivatives works are highly litigated and there is case law that you could turn to to help you make this decision but based on your example its going to be a fine line. You would have to be able to distinguish the products as two very separate works. Since the original book Z is inspiring your new book this may be hard to do. It might actually be easier to talk with the original author about your future plans and develop an agreement between the two of you where he waives any claims he might have here in exchange for recognition in your book. Just an idea--FYI- this is NOT LEGAL ADVICE. You should consult with an attorney in more detail here.

The question would require you speaking to an attorney to discuss the matter further in order for you to get any sort of guidance on this issue.

This is a difficult question that requires specialized and specific advise. The problem being that it is different in different jurisdictions and in different instances.

On its face, you would be generating a derivative work if it derived from what you learned and taught in the program - hence derivative. Unfortunately (or fortunately for you) this is often seen as a limitation on future "work" and as such as a non-compete of sorts. Thus, the issue is clouded by how much is derivative work vs. how much is you doing your job and growing and learning in that job. To complicate matters further, jurisdictional variance is high, so where this happens matters.

In essence, I would say that if you device the next plan for X based on your teaching and certification in X, then it is likely derivative. That does not mean that the clause is enforceable (depending on where you are and where the clause is governed). If you generate a program for Y based on experience in X but unrelated to X, then it may or may not be a derivative work. If you develop plan Y because X does not work and do not rely on any of X, then it is unlikely to be derivative in nature.