"Harry Potter and the Order of the Phoenix" is another great Harry Potter book that I loved a lot. I know some people think Harry is too angry in the book, but it didn't bother me. He was constantly in tough situations with basically no help. I'd be angry, too.
The villain of the book is Dolores Umbridge, and she was really good. I think part of the reason she's so great is because she's nuanced. She's not portrayed as 100%, flat-out evil like Voldemort is. Some of the things she does make sense. Like, yes, Hogwarts DOES need someone to supervise and monitor the faculty! Hagrid and Trelawney are both incompetent. Snape knows his material, but he's a horrible teacher who treats students like garbage, and he only got the job because of a personal favor to the Headmaster. In fact, "personal favor to the headmaster" is the only reason Trelawney got her job. Sorry, Headmaster Dumbledore, teaching positions are not trading cards. You can't give them out like lemon drops. There should be a vetting process.
Also, Hogwarts has standardized testing at the end of years 5 and 7. They really should have standardized lesson plans. It's not helpful if Professor Hagrid makes things up as he goes along. He should teach to the test.
I guess I'm trying to say that Umbridge is misguided, more than evil. Also, Dumbledore is a bit misguided in this book. His strategy is "keep everything a secret from Harry", especially all the important stuff that Harry NEEDS to know. If Dumbledore had been honest with Harry from the beginning, hey, the big fancy ending sequence would never have happened. Harry would have seen through the villain's trick instead of falling for it.
Another thing I liked about the book was the plot. It felt like there was more plot than normal, and things were constantly moving and happening. That was nice, and major props to the movie for keeping a lot of the plot elements instead of eliminating them wholesale. I thought Luna Lovegood was an interesting new character, although sadly, this is pretty much the only time we'll get to see her.
Oh, and romance! Harry gets his first kiss in this book, and all the fans went crazy.
Overall, a great book.