Now, slight gripage here.
4. One series, one cover style
To all those publishers who decide to change the cover style of books in a series partway through the publication of a series, I both despise and admire you.
I despise you because I can’t stand having books on my shelves that are in a single series but have multiple cover styles. Puffin, you are particularly guilty of this, but the rest of you are not exempt from the circumference of my bile. What’s the matter? Too much marketing budget? Too much boredom amongst the design department? Whatever your excuse is, I’m sure it’s both lame and unnecessary. It is vexing. You are VEXING me. The sight of a line-up of coherently designed titles is internally satisfying to me and promotes healthy and generous emotions towards publishers. The sight of a mishmash of different styles and, horror, different sizes forces me to think incredibly ungenerous things in your direction. Stop it. Stop it NOW.

I love this duo, Eon and Eona. But why, oh, WHY are the two spines totally inconsistent?

I haven’t even been able to bring myself to buy the fifth book in Anthony Horowitz’s The Power of Five series. A massive hiatus between books four and five naturally meant a total cover overhaul for book five.

Just no. Cover overhaul and a size difference. This makes my eyes hurt.
So, still, stop it. Stop it NOW.