Keep in mind that these books have nothing to do with RPGs, they are books of belief. Witchcraft is a real religion and people take their religious beliefs seriously and personally.
These books have been helpful for a variety of reasons. If for nothing else to get me to think about witches differently than other game authors have.
Encyclopedia of Witchcraft and Demonology by Russell Hope Robbins
This book has been long out of print, and I got mine at a used book store, but it is the indespensible work on witches, the witch craze and demonology. The book takes a very pro-witch point of view as it frankly discusses the murder of women, children and even men in the name of god. Not to be missed, this book has been THE source for most of my writings. Several editions are out there, mine is the 1959 edition. I have seen them on Ebay as well.
The Encyclopedia of Witches and Witchcraft by Rosemary Ellen Guiley
It would be unfair to compare this book to the work above it, because this book is very good. More a modern spin, it still deals with a variety of topics that are of interest to the witch or occult scholar. I used this book a lot when writing Eldritch Witchery. This book also has the advantage of being much newer and still available. All around a good mix of topics, witch myth is mixed in with modern Wicca, making difficult for the unlearned to know what is what. But for those who know a bosom from a common broom, then you will enjoy this book. If you have your own Bosom, then you might even find this book a little simple, but I found things there I did not know about, so I enjoyed it!
Another encyclopedia This one though covers less but goes deeper on it's topics. I like Buckland's writing style more than some of the others listed here. Gave me a lot of insight on various witch traditions.
Encyclopedia of Wicca & Witchcraft by Raven Grimassi
Another encyclopedia of all things witchy. This one focuses on more modern practices, has a very large list of contacts, email and websites. Some overlap with Guiley's book. If you are interested in the nature of the religion of witches today, then this is your book. Or at least it is a good place to start.
Covencraft : Witchcraft for Three or More by Amber K
A nice a well written book for witches and lay-people (cowans) to understand what a coven is and does. This is not a primer to witches, witchcraft or wicca, this book assumes you know what you need to know and works on what you may not know. It is concise and intelligent and a good read, even for non-witches. Maybe especially for non-witches, so that others can finally learn what witches do. From Llewellyn Publications.
Witchcraft, Sorcery and Superstition by Jules Michelet, A. R. Allinson (Translator)
A good book that deals with the wicth craze of the middle ages. Discussion range from early "faerie stories" to pagan religions, to the church sanctioned murder of thousands. Sometime muddled, the book has a few rare gems. Plus it delves into the socilogy of fear, why these people did what they did. Gets as far as the Salem witch trials. Very little to do with modern witchcraft.
Cassell Dictionary of Witchcraft by David Pickering
Another in a series of encyclopedia/dictionaries about witchcraft. A very good beginners guide.
Witches by Erica Jong
More of a coffee table book than an indepth treatsie on witches or witchcraft. Heavy on the feminist side, but not man-bashing (it is an Eric "Fear of Flying" Jong book) it is visually stunning. An oversized book, so if you look for it in the library you might have to check the oversized selves. My local library cut out some of the more risque pages. So I bought my own copy.
The Modern Witch's Spellbook, Book II by Sarah Lyddon Morrison
I read this, along with Book I, years ago, towards the first drafts of my witch class. It has spells that can supposedly really cast. Never tried. But it is a good insight on how some witches view or can view magic. A picture of me reading this over the top of my glasses (Thomas Dolby style, there THAT's long ago it was!) made my High School newspaper!.
Man and His Symbols by Carl Gustav Jung
I alway's liked Jung. His psychoanalytic theory always sat better with me than Freud's. This book deals with many of the archetypes we deal thing at an uncounscious level, the witch is one of many type of female, or anima, archetypes. Jung is one of the greatest thinkers of the modern era, and this is one of his masterpieces (along with Synchronicity).
Drawing Down the Moon : Witches, Druids, Goddess-Worshipers, and Other Pagans in America Today by Margot Adler
This is the book on modern pagan practices. Since it deals with a real world religion i only used it as inspiration to the Witches Netbook I wrote. But if you want to know what is going on in the world of paganism today, then this is your first stop. It is very telling for the foresightedness of this book when you consider that it was first published in 1979!
To Ride a Silver Broomstick : New Generation Witchcraft by Silver Ravenwolf
Similar in nature to Adler's book above, this book deal with modern wicca and witchcraft. Ravenwolf is very much a witchcraft supporter and her point of view shows through in this book, for good and ill. A good place to start and then return to when you know a little more. Generally speaking I am not a huge fan of Silver Ravenwolf, but at the same time I am not really her target audience, so my opinion should be taken with a grain of salt.
She has similar fare in her other books, To Stir a Magick Cauldron and To Light a Sacred Flame.
The Penguin Book of Witches & Warlocks : Tales of Black Magic, Old & New editied by Marvin Kaye
I have enjoyed the anthologies of Marvin Kaye for years. Ghosts, Vampires, Demons & Devils, and now witches. I found this in the library and had to get a copy from Ebay. It is a collection of short stories about witches, warlocks and witchcraft. The quality varies, but gems are true gems.
I will post more soon.