Tighter than the first two, darker in all the right ways, and the twist lands hard even on a reread. Rowling is clearly having more fun here. You can feel the series opening up into something bigger.

by J. K. Rowling
Harry Potter, along with his best friends, Ron and Hermione, is about to start his third year at Hogwarts School of Witchcraft and Wizardry. Harry can't wait to get back to school after the summer holidays. (Who wouldn't if they lived with the horrible Dursleys?) But when Harry gets to Hogwarts, the atmosphere is tense. There's an escaped mass murderer on the loose, and the sinister prison guards of Azkaban have been called in to guard the school...
Tighter than the first two, darker in all the right ways, and the twist lands hard even on a reread. Rowling is clearly having more fun here. You can feel the series opening up into something bigger.
The best book in the series, and it's not close. The time travel mechanics are airtight, the new characters (Lupin, Sirius) are the most interesting adults in the whole series, and the tone shift from the first two books is handled perfectly. This is where the series stops being for kids and starts being for everyone.
Lupin is one of the best mentor characters in fantasy, full stop. The way his arc is set up and paid off is masterful, and the Marauders backstory adds so much depth to the world. If someone tells me this isn't their favorite HP book I genuinely want to hear their argument.
Taylor Jenkins Reid