Rare witchcraft books can be hard to look into because so many of them are actually fictional. But if you are looking for some seriously hard-to-find- books on witchcraft, here are a few ideas.
Before I start, I do need to address the fictional book issue. The main one that people still intrigued with is the Necronomicon. That's the biggest book hoax out there. I have a whole other page about that though. But from the same Lovecraftian source as the Necronomicon, you may also have heard of "De Vermis Mysteriis" or the "Unspeakable Cults (Unaussprechlichen Kulten)" which are also very fictional.
You can also find that original copies of some witchcraft books are very rate even though modern day reproductions are easy to get. Here are a few that would fall into this category:
The Malleus Maleficarum - Published in 1486 as a manual on hunting and punishing witches. A historical classic but not really about witchcraft.
High Magic's Aide - The first novel written by Wiccan founder Gerald Gardner. Though fiction, it includes many accurate aspects of early British witchcraft.
ARADIA: Gospel of the Witches - This is another great source book that looks into the origins of Italian witchcraft.
You also need to remember that Wicca and "modern witchcraft" is fairly new so you aren't going to find books on the subject that are hundreds of years old. They just don't exist. Most really old witchcraft books focus on more general areas of the occult, alchemy, demonology and similar topics.