Severus Snape's copy of Advanced Potion-Making

"This Book is the Property of the Half-Blood Prince"

- Severus Snape's signature in the book

Severus Snape, also known as the Half-Blood Prince, owned a personally-annotated copy of the book Advanced Potion-Making.

Under Harry Potter's possession
About twenty or so years later, Harry Potter took N.E.W.T. Potions under the newly reinstated Slughorn. Harry did not buy his own textbook, as he believed that he indeed failed to actually qualify for advancement. As such, Slughorn loaned one of the older books left behind. Harry, by chance, got a hold of this book. While annoyed at first by the extra strange scribbles that made it difficult to read the original instructions, when Harry tried Snape's methods, he achieved extremely better results than even Hermione Granger. When Harry got a new book from Flourish and Blotts, he then swapped the front covers and gave Slughorn the new one with a combination of the Severing and Mending Charm, while keeping this one for himself, much to Hermione Granger's immense displeasure.

"If it hadn't been for the Prince, Ron wouldn't be sitting here now."

- Harry on the book's help in saving his best friend's life.

For the rest of the school year, Harry became a much better potioneer than he had been in his time with Severus Snape and even learned to save Ron Weasley's life with a bezoar when the latter had been poisoned. This book also helped Harry to win a bottle of Felix Felicis during his first class, which became incredibly essential in helping him acquire a memory from Professor Horace Slughorn about Tom Riddle and Horcruxes.

"He felt stunned; it was as though a beloved pet had turned suddenly savage. What had the Prince been thinking to copy such a spell into his book?"

- Harry Potter on the book after the effects of Sectumsempra were shown.

Over the year, Harry soon tried the Half-Blood Prince's self-invented spells written in the textbook, often with hilarious results. However, when he used Sectumsempra on Draco Malfoy, he was unaware of its effects, and Malfoy was critically injured. Nevertheless, Harry was severely shocked when he saw the deadliness of the curse.

When Madam Pince saw the book with its highly graffiti-covered pages, she flew into a rage, despite Harry claiming that it was not the library's. She clawed the air for it, and throwing angry claims at Harry for ruining it with the writing, all the while looking as though she would have a seizure.

During one of Slughorn's Slug Club parties, Snape was told of Harry's sudden improvement in the art of potion-making. Snape then suspected that his old textbook may have fallen into Harry's hands. When Harry used Sectumsempra on Draco Malfoy, Snape was able to confirm his suspicions, but was unable to get his hands on the book, as Harry had stashed the book inside the Room of Requirement and used Ron's book as a decoy.

"If you had a shred of self-respect, you'd hand that book in."

- Hermione Granger's resentment in the book.

Hermione Granger resented the book, for it did not follow the "official instructions." She did some research, but the only information she found pertained to Eileen Prince. Later, Snape revealed that he was the Half-Blood Prince, and that he owned the book before. This came as a major shock to Harry, when he learned that the Half-Blood Prince, who had taught him so much over the year, happened to be the person he hated the most (especially after Snape killed Albus Dumbledore).

It is unknown if Harry removed the book from the Room of Requirement after, but if not, then it was most likely destroyed when Vincent Crabbe released Fiendfyre in the room during the Battle of Hogwarts.

Altered Potion formulae

 * Draught of Living Death
 * Crush the Sopophorous bean with flat side of silver dagger, releases juice better than cutting.
 * Add one clockwise stir after seven counter-clockwise stirs to obtain the clear colour faster.
 * Poison Antidote
 * Simply shove a bezoar down the victim's throat
 * Elixir to Induce Euphoria
 * Heavily corrected
 * Add a sprig of peppermint to counter excessive singing and nose-tweaking side-effects.

Invented Spells

 * Langlock
 * Levicorpus (non-verbal)
 * Liberacorpus (counter-spell for Levicorpus)
 * Muffliato
 * Sectumsempra ("for enemies")
 * Toenail Growth Hex

Behind the scenes

 * Of all of Snape's spells found in the Potions book, only Sectumsempra appears in the film version of Harry Potter and the Half-Blood Prince.
 * In the film version, when Harry and Ron saw this battered book and a better condition one in the cupboard, they fought over the better one, with Ron winning it, leaving Harry with this.

Appearances

 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Half-Blood Prince (video game)