Harry Potter And The Half-Blood Prince by J. K. Rowling
Harry is now sixteen. Back at Hogwarts, he worries that his rival Draco Malfoy has become a “Death-Eater” agent of Lord Voldemort. No one else takes his concerns seriously.
He spies on Draco and follow leads while Hermione and Ron bicker before realizing their mutual attraction. Dumbledore gives Harry private lessons that center on showing him some memories relating to Lord Voldemort’s past.
The Half-Blood Prince is the owner of an old potions textbook that Harry uses, and the Prince’s notations in the margins help Harry but also grow more and more sinister. Harry is also dismayed by his sudden attraction to Ginny, the younger sister of his best friend Ron. (Unfortunately, there are also descriptions of two Quidditch matches.)
The excitement really ignites when it becomes clear who the Half-Blood Prince is. The sequel (and last book in the series) should be intense