Freebird, the Society of Authors has an excellent booklet on publishing contracts, which will tell you what you need to know: it's not straightforward stuff (which is why I'm hesitating to say anything specific), and, as you've realised, something like your commission may be a bit different from, say, a novel that's already finished. The booklet's £10, or free to members:
http://www.societyofauthors.org/publications/
But you might want to consider joining the Society: you're eligible if you've been offered a publishing contract, and you can send the contract in with your membership fee. They'll go over it with their legendary fine-toothed comb, and get back to you within a few days to tell you about anything really bad, but also explain anything which isn't bad, but which you might not have realised the implications of. They're then a brilliant source of advice and networking for the rest of time...
http://www.societyofauthors.org/index.html
Emma