I know it's the contents that are important, but series are rarely more than five/six books. Would it be that hard to keep them the same design (or even size!)?
It's my Artemis Fowl ones that are annoying me today (I've been re-ordering my shelves). They're all first editions, so the cover that each book started off with. The first three are the same, the fourth a bit different and then after that they are a completely new design, it almost looks like a completely different series. The author's name goes to a completely different place too.
Lots of series I have are a mixture of paperback and hard back, but that doesn't annoy me nearly as much as long as the style is consistent!