I don't know about you, but I like the series I read to look like series. I used to be batty about them being the same size, but I got over that. I figure if I need to read it, I'm buying it no matter what size it is. The cover art, though... It has to look like a series for me. Amy Plum's Revenant series got it right:
These covers go together. Theme, color saturation, font, filigree. All of it. They're gorgeous on their own, and stunning altogether. This is a series done right. Another couple exmples: Leah Clifford's A Touch series, and Kiersten White's Paranormalcy series.
It really makes me cranky when a publisher changes the theme/style for a series' cover in the middle. You expect them to look similar, like the examples above, so you know what you're looking for when you go to the bookstore, so they present a single theme. It really throws me off when covers change, when the cover artists, or marketing department makes a change that really seems to jump the shark. I can't be the only one so nutty about covers, right?
For example, the Nightshade series, by Andrea Cremer:
Then you have the cover for the sequel:
Do the two go together? Not to me. Not at all. It make my image-oriented nit-picky brain kinda twitchy.
How about another example from a very popular YA series?
Then, the sequel cover was released. I was amoung the hundreds waiting, wiping foam from the corners of my mouth. And the cover did not disappoint:
After these covers I was dying for the third. Imagine my shock and, yes, diappointment when the cover for the final book was revealed...
So not like the others, it was almost a smack in my panty-drooly-gimme more face. Where's the wistful colors, the sense of yearning in the models? Wait! Where are the models?? Sure, the vast majority of this story takes place on a planet, not a spaceship, but it's just so DIFFERENT. Nothing similar in composition, style, color, not even font, and no models. Yes, it fits elements of the novel, but I do not believe it fits the story as well as the others fit theirs. (I still liked the story...in case you were wondering. I had to have it despite the mismatched cover)
Now, when you look at all the paperback versions of this series, the three have a definite theme and continuity, all close to the cover above. I feel a tad cheated I didn't get a pretty third cover, and I know if I hadn't seen the original cover, I would've missed out on an amazing series.
What about you? Are you nutso about cover art continuity? Does the cover image matter to you? Do you have examples of cover series gone wrong?