Hi,
I can't speak on contracts because I have worked with self publishers on book covers, album covers, and interiors for chapter books but always faith based. I was never screwed over so that's lucky and I'll definately use contracts from this point on.
Here's some things I learned that may be helpful.
~Get as much info about the kind of drawings, how much detail they want, agree on how far you're willing to take the art beforehand because you don't want to agree to a price and find out they want a lot more than they originally asked.
~Ask how much creative control you have. You want to get portfolio pieces from it as well as payment. Also probably want to make sure that there vision for the project is an art style that you'll enjoy making.
~I've heard some people set limits on the number of revisions. I had someone come back to me after 6 months for a revision. I did it without complaining because it was an easy fix but I wasn't excited about making the changes.
~If the budget is small, consider offerering simpler versions of what they want. Like black and white or simple spot illustrations. That's if you really want the job.
~I would ask what piece of yours they saw and liked so that you have an idea of what they are looking for.
~Negotiate for half up front if you feel that's right.
~Most of all, get all the details of the job first. I know I said that but a few projects turned out way bigger because I didn't know how to interview clients.
So this is a lot, and I am an amateur with little work experience but these are things I wish I had done when I worked with self publishers. Will Terry and Tyrus Goshay have a really good video on youtube about working with self published authors.
Good luck!