In the post credit scene in the final episode of WandaVision Season 1, Wanda (Scarlet Witch) is seen reading a book. What's the origin behind the book & what's it called?
The book is the "Darkhold," or "the book of the damned," as mentioned earlier by Agatha when she is reading from it.
Agatha: Wanda, you've never been up against another witch before. Did you know there's an entire chapter devoted to you in the Darkhold? That's the book of the damned. "The Scarlet Witch is not born, she is forged. "She has no coven, no need for incantation."
WandaVision, Series 1 Episode 9, “The Series Finale”
Apart from that, per the MCU we don’t really know much about it; it doesn’t show up in Doctor Strange if I remember correctly. It seemed Agatha was able to conjure it up though, so presumably Wanda got the book in the same way.
The book does show up in Agents of S.H.I.E.L.D. and Runaways, but as the canonicity of those shows to the MCU is a little debatable at this point, and I haven’t watched them, I’m not going to look into it here.