Harry Potter and the Order of the Phoenix

J.K. Rowling switches gears in the Harry Potter Series once again.
The 5th book is the longest book in the series. Which is not a bad thing, although some parts seem to be dragged on a bit too much(looking at you, Umbridge)

This book is the point where the series culminates into something larger. And the Harry Potter universe is established properly, especially with plenty of references to past events(most before the first book) which impact the current world in the book. These are possibly a good way to sow some seeds for future prequels.

Also, a lot of the events from previous books are connected and the stage is set for the next books. Many of the other characters(who had small roles to play in earlier books) have been developed well too.

Overall, this book actually gives a glimpse of the depth of the thought process that JK Rowling has about the whole Harry Potter universe. Which is one of the main reasons that the whole series is loved so much by everyone.