if it is being done by a "real" publisher (you know what i mean), and especially if the art is print-size (100%), please do NOT do 300dpi. that works for printing sketchbooks off a laser printer etc, but for professional offset printing from some printing plant in china, it's not good enough. (i'm not trying to be a dick, i've done sketchbooks and kid's books and comics.) ideally you should do 600dpi. if that's too big for your computer, and the art style is more painterly (not crisp, inked lines) then you can get away with generating the art at 450dpi, then taking the final, flattened art up to 600dpi to save as a final file.
also keep in mind that there might be promotional and other supplemental material that blows up the art, so you want it to be better than the minimum. and likewise, it's not unusual to reuse art at a different scale for the title page, back cover etc. oh, and the publisher's catalogs, too!
and good luck/congratulations!