The Gothique Library & Spell Book
The spell book is one of the first "extra actions" items for sale in the Sims 3 online store. You can buy the Lady Ravendancer Goth's Book 'o Spells as a stand-alone item (500 Sim Points, or $5) or as part of the Gothique Library set (1,200 Sim Points, or $12).
I bought the entire set, primarily because I was curious about the Gothique Library that comes with it. The description made it sound like it might have a secret tomb or adventures or something.
I installed the library in my town and did a quick investigation, and it seems like this is not the case. It's pretty funny to have the world's first combination library and cemetery, though! Even if the game did name it "2" for some unknown reason (not "Library 2," just… "2".)
The spell book, which rests atop its own podium just like in Sims 2, is the obvious attraction here. Once you buy the spell book you will find it in Buy Mode -> Skills/Hobbies -> Misc. It costs your Sim a whopping $72,000, making it the second most-expensive in-game item. (the F-Class Hunter Killer comes in #1 at $75,000.)
The spell book allows you to learn five spells:
Summon: Magically teleports a random Sim to your location.
Summon Acquainted: The same, but it lets you choose which Sim to bring over.
Mood Elevatus: Supposedly this improves the mood of every Sim in the room. I've only had it backfire, though. But frankly, that's pretty funny in and of itself!
Unleash Animale: This gives every Sim in the room a -62 negative moodlet "Cursed" for 12 hours. Periodically the cursed Sims will get the "Unleash Animale" action in their queue. It looks like they scratch themselves, wash their faces like a cat, and growl.
Enchant: I haven't tried this, but it apparently "brings the spell book to life."
You have two options: Learn Spell and Review Spell. It takes a lot of practice to get a spell right, and if you get it wrong, there can be dire consequences. (Or at least embarrassment.) So you will probably want to Review a spell several times before trying it out.
Your Sim can only cast a spell when they have the "Makin Magic" +15 moodlet, which appears after they practice or perform a spell. To cast a spell, just click on the Sim who has learned the magic and choose "Cast Spell."