By day, I'm a domestic violence prosecutor. By night, I read romance to restore my faith in love, relationships, and humanity in general.
Of all the Harry Potter books, I think Order of the Phoenix is the most difficult to read. -Not because it's bad (as if), or because it is dark (it is, but there is plenty of mirth interspersed with the darkness), but because Harry is subjected to unrelenting injustice on all fronts. Harry is no stranger to injustice, of course: he grew up being mistreated by his Muggle guardians, the Dursleys. He's always been treated unfairly by Potions Master Severus Snape. At various points in his Hogwarts career, he's also had to suffer widespead ridicule and shunning by his schoolmates (most notably in Chamber of Secrets, when many thought he was the Heir of Slytherin, and again in Goblet of Fire, when his selection as the fourth Triwizard champion inspired the resentment and jealousy of many, including his best friend). In Order of the Phoenix, however, Harry is more persecuted and ostracized than ever. For reasons he can't understand, his mentor, Albus Dumbledore, won't speak to or even look at him. Also, in a bid to discredit his claim that the evil Lord Voldemort has returned to full power, the Ministry of Magic and the main wizarding news source, The Daily Prophet, are spreading the word that Harry is crazy, unbalanced, and possibly dangerous -- and some of his schoolmates are only too willing to believe the propaganda.
Worse yet, not only has the Ministry turned on Harry, it has infiltrated his home base, Hogwarts. When Dumbledore can't find anyone to take on the jinxed Defense Against Dark Arts teaching position, the Ministry installs the cloyingly sweet and noxiously poisonous Delores Umbridge. Umbridge is my personal favorite villian in a series with a lot of baddies: she is maddeningly, gut-churninging, teeth-grindingly NASTY. I hate her. Harry hates her. Hermione hates her, teacher's pet though Hermione usually is. Even Minerva McGonagall, who has always been crisply polite (though occasionally cutting) to even the most incompetent fools (see Gilderoy Lockhart), hates Umbridge and cannot keep a civil tongue around her.
As the year progresses and the Ministry passes new laws to increase Umbridge's authority while undermining Dumbledore's, Umbridge strips joy from Harry's life wherever she can. She tortures him in detentions. She bans him from quidditch. She intercepts his mail and watches the fireplaces so he can't communicate with Sirius, the closest thing to family he's got. Eventually, she banishes those teachers who have always provided Harry with comfort and guidance.
Never has Harry been so alone, and this extreme isolation comes at the worst possible time. Harry has always been able to trust his instincts in the past, but now even his thoughts are untrustworthy. Not only is he full of the usual teenage angst and anger, but Voldemort may be able to infiltrate and influence Harry's mind. The Ministry is trying to convince the world that Harry is crazy and evil, and suddenly, deep down inside, Harry can't be certain that he isn't.
This is probably the darkest book in the series. It's early days in the war against Voldemort and the Death Eaters, and there are greater losses yet to come, but this is the book where Harry's faith in himself is at its nadir. Until he overcomes this crisis of confidence and learns to master his own mind, he cannot begin to fight the battles that will come.