Harry Potter and the Half blood Prince - it was somehow written differently than the other books and had a unique air and mood to the story line. Harry's character had developed well and so had the other relationships. There was a feeling of lightness and bouyancy at Hogwarts (Potions and Quidditch), interspersed with surprising insights into Voldemort's past, the weighty knowledge of the Horcruxes and so much more. Dumbledore's death and Snape's revelation as the Half blood Prince steered the entire story to a completely new place - so unexpectedly. The ending was tinged with so much sadness, desperation and hope...it was my favorite book of the series.
I read it within 8 hours and spent the rest of the day in a half trance. Best reading experience ever!