If you are lucky enough to get to work with the same author on a book series one of the things you might want to think about first is Cover Continuity. Most book series are written with a over reaching story arch for the entire series.....meaning all the books in a given series might have a continuing set of characters and an ongoing time frame or period for the stories to take place. When you work on book covers for a series it's up to you, the illustrator, to continue that series' “visual identity.”
In an ongoing series of middle grade books by Rosalyn Rikel Ramage, The Tracks, The Graveyard and The Windmill, I've kept the same feel....slightly dark.....and some of the same characters, siblings Emma Mae, Edward front and center on all three covers. And by the author's request all three covers feature “transition” points in all three stories, where the “real” story events make a transition into a bit of fantasy.
Look at all three covers lined up above......they all seem to “hang” together with the same kind of feel and design.
For the next two covers, Bo and the Roaring Pines and Bo and the Christmas Bandit, I used the main character Bo, a young black lab....hero of these middle grade books from Pelican publishing written by Lynn Sheffield Simmons. Bo's all black shiny coat makes a wonderful foil for the brite yellow cover background. That color was suggested by the author. She said that over many author signings and school visits, she'd tried out different backgrounds for posters....and found that the brite yellow color attracted more attention than any other color. So she suggested that yellow as a “branding tool” for her Bo series of books.
Visual branding is a valuable tool that shouldn't be under-estimated in today's “battle for consumers' eyes” and dollars.