You already know the answer. So ask for the compensation you negotiated for and, if it's not forthcoming, then speak with your own attorney about how to collect. Press on.
The above is general information ONLY and is not legal advice, does not form an attorney-client relationship, and should NOT be relied upon to take or refrain from taking any action. I am not your attorney. You should seek the advice of competent counsel before taking any action related to your inquiry.
In principle you should be compensated for your authorship, expect that compensation does not necessarily means money. The question you and your copyright attorney need to answer is whether the terms/contract whereby your art was used was a license, a donation, work for hire. What were the terms regarding ownership and subsequent rights. Contracts need not be in writing, they may be oral.
A Copyright attorney needs to comb through the history of your relationship and build a case to determine whether compensation is due and rights in your art for derivative works.
USPTO Registered Patent Attorney, Master of Intellectual Property law, MBA I am neither your attorney, nor my answers or comments in AVVO.com create an attorney-client relationship with you. You may accept or disregard my free advice in AVVO.com at your own risk. I am a Patent Attorney, admitted to the USPTO and to the Florida Bar.
Yes, of course, if you did not agree to do it for free. You don't get paid just because you want to get paid or "should" get paid. You get paid because you cause the payor to pay. A lawyer can likely help make that happen. You did not say what your deal, if any, was on the book you illustrated. That makes me think you might have agreed to not get paid or might have done this as an employee or signed a copyright assignment. You really need to see a lawyer and give the whole story, not this partial one-sided, vague version you gave here. Provide additional facts, if you want more from us.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.
As the creator of the illustrations, you are certainly entitled to compensation - at least for the recent usage. If you have not yet found an attorney in NJ to help, contact me - in a 1/2 hour free consultation I can at least put you on the path to registration of your illustrations with the U.S. Copyright Office. If you have already an idea of what you think is reasonable compensation, we can discuss what to do next.