All book deals should have a written agreement between the publisher and illustrator (or author/illustrator) in place before any work is started on the book content. It is not a case of agreeing to any contract offered to you, but assessing the details of the agreement and discussing them with the publisher – negotiation is completely normal. This resource gives the basic considerations for such a situation.
If you want more information, an in depth resource on Negotiating Contracts with Publishers is available for AOI Members.
AOI can advise members on their contracts. Email us with details of the commission and publisher and any questions about the contract.