ANSWER: You could try talking to her? I really wouldn't panic, very few witches are going to give a thirteen year old girl access to any magic that is likely to do her harm. Chances are if she is reading books on Witchcraft its pretty general stuff of an informative nature. It may indicate a personal interest in witchcraft, it may just indicate an interest in witches as a subject matter. Many young people will try out/study a range of different belief sets before finding the one that is their true calling.I may sound a bit harsh, but have you thought about allowing her a little privacy? If you learned to communicate better, you may not have to spend your time going through her room and finding things you obviously don't like. Teenagers need their space and privacy as much as the rest of us and in denying her that, you are perhaps pushing her further away from you.
Broach the subject with her but be open minded, possibly read a book or two yourself first. Witchcraft is not the dark craft it is portrayed as in the media. You may even find it of benefit if you do end up with a witch in the family. But if you go in all guns blazing telling her what she is doing is wrong, you run the risk that she will end up practising covertly and then you will have no knowledge of what she is doing and more importantly who she may be doing it with. If she feels that you can discuss the subject of witchcraft with an open mind and common sense, she may well be more willing to share with you what she is doing and where her beliefs are taking her.
I wouldn't worry too much about the black candles either. The association isn't what you think. Black is more commonly used for protection in candle magic than for anything else. Have a look at the colour correspondences on my site.