In these chapters, things are looking pretty dire. McGonagall is in the hospital after she was injured trying to help Hagrid during his removal from school grounds. Therefore, when Harry has a vision of Sirius and Voldemort in the Department of Mysteries, he can't go to her with his concerns. He tries to check on Sirius using the fireplace in Umbridge's office, and is caught. Only after he and Ron and Hermione lead Umbridge into the forest to be dealt with by the centaurs can Harry even consider pursuing a rescue mission, and of course when he does, Sirius ends up coming to rescue him and losing his life in the process.
Unfortunately, because I didn't read this book right when it came out, a kid at the summer program where I worked at the time told me about Sirius's death before I got to read it for myself, so I've never had a truly unspoiled reading of this book. Even so, I can tell by the way it is written that it was a real shock to those who were unspoiled. There is no preparation at all, aside from one little line of foreshadowing where Harry says he hopes Ron won't be able to see a thestral. Otherwise, the moment of his death is so sudden and out-of-the-blue that it takes a moment for the shock to set in.
A few other things caught my attention as I re-read. For one thing, Umbridge's encounter with the centaurs was a lot darker than I remembered, and I felt very uneasy during that scene. I had also forgotten about the fact that Voldemort was trying to use Harry to get a prophecy from the Department of Mysteries, and that struck me as pretty lame, considering all the danger and high drama involved in getting Harry there. I was also really irritated by Rowling's use of Ron as comic relief during the battle. I mentioned this when I reviewed Harry Potter and the Cursed Child - Ron is my favorite character, and he deserves to have serious moments. (I look forward to re-reading his destruction of the locket in book 7.) Finally, I really felt for Hermione, who basically told Harry his dreams were leading him into a trap. How awful it must be for her to know she was right and couldn't convince anyone else.
Only three chapters to go. I'll finish the book this week, and then on to book 6!