The subject title does give away what this post is about. Alongside the current title I'm writing, I'm trying to build up a proper fantasy world and would like to do further research than I've done previous for any writing project. I don't have much in the way mythology and witchcraft ideas go, so I'd like to know if there's any books that goes into deep, deep detail of either subject to help. Edit: Due to an additional thought I've come to, while I already now a fair amount to the subject of greek mythology (thank you popular media) the matter of that is optional to be brought into any books that detail mythology. Though if the book mentions something that's not commonly known, I'll be all for it.
If you'd like to read about wise and powerful women in Celtic mythology, I suggest you get a hold of a copy of If Women Rose Rooted: A Life-Changing Journey to Authenticity and Belonging by Sharon Blackie.
I wouldn't put my faith in the portrayal of Greek mythology (or any mythology, come to that) in popular media. For Greek, you could do worse than read the Iliad and Odyssey. For Norse, the Prose Edda. Egyptian, perhaps the Book of the Dead. Indian, either the Mahabharata or Ramayana. Japanese, the Kojiki and Nihon Shoki. Demonology, the Ars Goetia or Daemonologie. Arthurian, Le Morte D'Arthur. None of these books are easy going though.
I would add Ovid's Metamorphoses, which is available on Archive.org. Many of these probably are. Then I'd also add The Greek Myths: The Complete and Definitive Edition by Robert Graves. Either that or Mythology: Timeless Tales of Gods and Heroes by Edith Hamilton.
Neil Gaiman's Norse Mythology is an easy read, and it's quite good, perhaps not the most comprehensive, but he brings the characters to life. Bonus: the audiobook is read by the author, which in Gaiman's case is a real treat. If you're looking for fiction that deals with mythologies you might not know, try Lost Gods by Brom. It's in my top twenty books of all time. It's so good. It's a dark fantasy in which the MC descends into the underworld where not only dead humans reside but also gods of dead religions. I can't speak to the validity of the mythologies referenced. In fact, I know at least one is made up because he encounters a god worshiped by Neanderthals. Either way, it's full of incredible worldbuilding and extremely clever use of mythology. Bonus: The print edition is beautifully (if disturbingly) illustrated by the author, who's been creating art for major movies, video games and all sorts of stuff since the late 80s.
