J. K. Rowling. Harry Potter and the Half-Blood Prince. Book 6

The Ministry of Magic is suffering defeat after defeat in its battles with Voldemort. Even Muggles are affected by the second war — every day disasters occur, their causes unknown to anyone. The Order of the Phoenix is losing its best wizards in the fights against the Death Eaters.

But even in times of war, life goes on. Harry, Ron, and Hermione, like all sixteen-year-olds, attend classes, go to parties, and fall in love. Yet danger draws ever closer. Despite Dumbledore’s best efforts to protect the school, very strange things are happening at Hogwarts.
This time, the Pensieve will reveal to Harry the greatest secret of the Dark Lord, and in order to win, Harry will have to sacrifice what he holds most dear…

But which side will the mysterious Half-Blood Prince choose in this war?