I'd hesitate to call them pet peeves, more like "things I loathe with a passion."
Books that misrepresent their genre or abruptly change gears halfway through. I just feel cheated, especially because I have yet to see it done well.
Historical inaccurancies: accidental ones are bad, willful and deliberate ones are worse and will not be forgiven.
In-your-face modern attitudes in historical settings.
The author showing a character's goodness/"enlightment" down my throat.
Characters who don't do anything - you've probably run across them before. They are supposed to have a job or responsibilities, yet they feel like bad actors who just stroll on the stage, do or say their bit to advance the plot and then go back to waiting in the wings for their next turn. You never get the impression they're actually, you know, doing something
when they're out of sight.
Taking "show don't tell" too much to heart. For example, I ran into this in a book divided in four parts: part 2 and part 3 were an extremely long flashback about the life, death and miracles of one of the main character's father. These two parts weren't very brief, either, and so I got to trudge through page and pages without actually getting nowhere. It was very frustrating and boring.
And the worst thing is, most of these pet peeves come from a single book.
I ought to sell it, I suppose, but how can I inflict it on somebody else with a clear conscience?