Whatever you negotiate get it all in writing but more important than royalties - what rights is the author/publisher asking for?
Do they own the artwork outright (work for hire) where you are paid a fee and that's it?
Who (under what name) is the art being copyrighted by, the publisher/author or you?
What reproduction rights are they purchasing? 1st rights, ancillary products, etc.
Who owns the originals?
How are you credited in the publication?
How many "changes" are allowed before additional fees kick in?
Is there a kill fee?
Payment structure? 1/3 upfront, 1/3 upon pencil approval, 1/3 final acceptance?
Working with self publishers can be a mine-field. Have an attorney look over the contract or join the graphic artists guild and download a boilerplate contract from their website.