Over the years I have read and critiqued the books of a variety of authors. The following is a list of authors that I personally do not recommend as good sources on Wicca or Paganism in general.
Kisma Stepanich - her books were found to be plagarized from another author and the publisher was forced to pull them from bookstores. But you'll still find copies out there.
http://www.geocities.com/ferigold/truthfaery/index.html
Silver Ravenwolf: Encourages kids to lie to their parents about what they are doing, teaches curses and hexes, has a pretty strong anti-Christian bias. But for a good summary of the problems with her, take a look at this essay.
http://wicca.timerift.net/ravenwolf.html
Fiona Horne - Got into this for the publicity. She admits that she isn't even a Pagan. Again, here's an essay on this author.
http://wicca.timerift.net/horne.html
Raven Grimassi - I have known Raven on-line since back in the old Compuserve BBS days. Back then he was claiming to be Strega and looking down on all us Wiccan types because we didn't have long histories. Then the Strega came along and pretty well forced him to stop using a title he hadn't earned..hence his writings are now called Italian Witchcraft instead of Strega. Now he has set himself up as an authority on Wicca but is buying into the myths of the 'ancient' religion and the Burning Times.
Ann Moura - Her books have some good information in them, I will admit. And her first two books were the best of the lot. But she tried to rewrite history to prove that Wicca is an ancient religion again! And she has a clearly anti-Christian bias in her books that bothers me.
D.J. Conway - Basically all her books are the same with the names changed to protect the guilty. Her books are full of inaccuracies, such as her book on Celtic Magic which discusses Karma and the Celts. Karma is an eastern concept and was not one that the Celts ever used. Her book on "Cat Magic" is nothing more than a rehash of every "how to care for your cat" book on the market with only one slim chapter on anything related to magic.
Edain McCoy - Also a purveyor of the Great Potato Fallacy. She is the inventor of 'Witta' which she tried to claim was the ancient Irish form of Wicca. It isn't. The name she called this tradition by is one that would not have been found in any Celtic language. It is simply Wicca given a made up name. The one book of hers that I thought was worth much at all is "Inside a Wiccan Coven" where she is writing about her own experiences and not about anything that took a bit of scholarship.
http://www.cyberwitch.com/wychwood/Library/whenIsACeltNotACelt.htm
Douglas Monroe - Very poor scholarship. His works on Druidry bear little resemblance to the known practices of the historical Druids, and even less to what is known about the Celts in general. Mr. Monroe is also a convicted pedophile.
http://www.davensjournal.com/index.htm?TOC.htm&1
Konstantinos - His books are a flagrant attempt to market to the Goth crowd. There is nothing in them that isn't covered better by other authors.
Gerina Dunwich - Lots and lots of fluff without a whole lot of substance. Many better authors out there to chose from, at least in my opinion.
Gavin and Yvonne Frost - Bad history and bad information. In addition, they recommend what amounts to nothing more than the sexual abuse of children in their books. They have stated that they are simply misunderstood, and they have also said that they won't discuss it because society isn't ready to understand it yet. But anyway you put it, the ritual defloration of young girls in ritual by their fathers is sexual abuse in my book.
http://st09.startlogic.com/~pendrago/goodwitch.html and
http://ajdrew.blogs.pagannation.com/defrost-please-read/
Sirona Knight: Has recently been accused of having plagarized much of the material in her books, particularly in her book "The Little Giant Encyclopedia of Runes."
I suppose one can learn something from almost any book, but the works of these particular authors require so much unlearning later because of the errors they contain that it is probably better just to spend your money on better books and save the time later to learn it right.
Hope this helps,
-Lark-