Not being able to keep a series looking in order in my bookcase annoys me. It annoys me when a series suddenly changes to hardcover. I know this is a good thing 'cos it means the author is getting more readers which I'm happy about but from a purely selfish point of view, it ruins my pretty shelves!
It also annoys me when a book comes out a lot bigger than the others. Mary Janice Davidson's Undead series and Sherrilyn Kenyon's Dark Hunter series come to mind here. It's not that the books are thicker or anything, they're taller and wider so look out of place on my shelves. The main reason this bugs me is that there's no reason for doing this. It means nothing and it also makes it look like a kids book because the writing is freaking HUGE in it!
The main thing that's bugging me lately is book cover designs getting changed midway through a series. That really pisses me off. Why the sudden need to change it if there's nothing wrong with it? This is in no way an author thing since not a lot of authors have a say in the cover.
One series springing to mind right now is Kelley Armstrong's Otherworld series. Book ten has not long been released and apparently the UK publishers think it's time for a revamp ... why? I know loads of people who LOVE the UK covers, even more so than their own country's covers so why the need for a revamp? There is none!
Publisher's suck sometimes!