Go to the library and enter the restricted section. Go to the very back and look to the left with your back to the gate that you entered from. Press X and there should be a scene when Harry says "The properties of Moonstone".
It's somewhere in the Library.
You get the ingredients and follow the instructions in the potions book.
Moste Pontente Potions
Horace, Horace Slughorn.
You can buy book #3 in the Harry Potter series at a public library or a bookshop/bookstore
Severus Snape is the potions teacher in most of the books. In the sixth book that position is taken by Horace Slughorn.
Harry used one of the spells in the book on Draco Malfoy and almost killed him. He no longer wanted to use the book and so hid it.
You can buy the book or borrow it from a library.
Severus Snape was born in 1960. He was thirty one in the first book.
Any good bookstore and or a library
Yes, the Harry Potter books. If you want more details about each spell and potions, please note that they are fictitious and won't work as in the books, so you can just as well make up your own spells and potions. In any case, there are references on the Web that collect all the details known from the books. You can find some things in the Wikipedia, or in the Harry Potter Lexicon.
Harry finds the book in the Potions dungeon storeroom when his Potions teacher Slughorn tells him to go ahead and take one of the old books.