you need to have your ownership of the copyrights to be spelled out clearly in writing as part of your agreement
it is all subject to negotiation, so perhaps you can work something out, but the ownership of the copyright needs to be clear because the default rule is that the creator is the copyright holder absent some other conditions (e.g. work for hire agreement, employer-employee, explicit contract)
so you need to work this out before proceeding
Should there be a release form between us from an attorney or can we do a contract which can be considered a legal document?
you can certainly write up your own agreement but it would be best if an attorney provided it or at least reviews the one you are planning to use.....to make sure the right language is in it and that all important aspects/rights are covered
You also want to out in the agreement/contract something about how the entire/full compensation for the artist is X, and they will have no interest in the books sales or other commercializations of the illustrations (like in other books, posters etc). But it's all upto negotiation and the the copyrights can be broken up as desired.....but definitely talk to a copyright lawyer because there are many aspects you probably haven't considered that could come up in the future.
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