Harry Potter and the Order of the Phoenix
I really liked books one through four. But this is the book that made me stop reading the series.
(show spoiler)
Also, Harry spends a lot of time speaking in caps lock. If an author can't show emphasis or shouting without resorting to writing dialogue in all capital letters, she needs to study the basics of writing. There's really no excuse for the sheer number of caps lock sentences. It made the book look like something written by a teenager.
The last thing I really hated was just personal preference on my part. Bad things happened to a character I liked.
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