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. At a young age, Snape was shown to be very talented in potions, and he made many alterations to the recipes and instructions in the book for better effects. In addition to these alterations, Snape also invented a number of spells, which he wrote down in the pages of the book.

Altered Potion Formula

 * Draught of Living Death
 * Crush the Sopophorous Bean with the silver knife releases more juice.
 * Add one clockwise stir after seven counter-clockwise stirs to obtain the clear colour faster.
 * Antidote to Common Poisons
 * 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.

Spells

 * Levicorpus (non-verbal)
 * Liberacorpus
 * Sectumsempra (for enemies)

History
When Snape attended Hogwarts, he acquired this textbook for his sixth year Potions class with Horace Slughorn. Snape, being a brilliant wizard even at a young age, has made alterations to the recipes within the book for better effects. He also made notes of spells he invented. When Snape graduated from school, he left the book in the potions classroom.

About twenty years later, when Harry Potter took N.E.W.T. Potions under the newly reinstated Slughorn, Harry didn't buy his own textbook because he thought he failed Snape's expectations for advancement. As such, Slughorn loaned one of the older books left behind. Harry by chance got a hold of this book. When Harry got a new book from Flourish and Blotts, he swapped the front covers and gave Slughorn the new one, while keeping this one for himself.

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

Over the year, Harry also tried the spells written in the book, often to hilarious effects. He thought the Prince merely copied them from other places, unaware they were actually his own inventions. However, when he used Sectumsempra on Draco Malfoy, he was unaware of its effects and Malfoy was critically injured. To hide the origins of his knowledge of the spell from Snape, Harry stashed the book inside the Room of Requirement and used Ron's book as a decoy. Nevertheless, Harry was shocked when he saw the cruel effects of the curse.

During one of Slughorn's Slug Club parties, Snape was told of Harry's sudden transcend in the arts of potion. Snape then suspected that his old textbook may have fallen into Harry's hands. When Sectumsempra was used, Snape was able to confirm his suspicions, but was unable to get his hands on the book, as Harry had used Ron's as a decoy.

Hermione Granger resented the book for it did not follow the "official instructions". She did some research and discovered that Snape was the Half-Blood Prince, and that he owned the book before. This was much of a 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 after, but if not, then it was most likely destroyed when Vincent Crabbe released Fiendfyre in the room during the Battle of Hogwarts.

Appearances

 * Harry Potter and the Half-Blood Prince