Well, I have to say that the best online spell book that you're going to find is this site right here. And while I may be a little biased, you can't deny that this is one of the best collections of witchcraft spells around.
Rather than send you to another online spell book, I'm going to take a minute to help you find your way around this one better.
There you have it. A complete map to navigating this online spell book. You shouldn't have any trouble finding the precise spell when you need it.
If you're looking for more links on the articles around this site, I have a new little index of spell articles that might help. Sometimes you need a bit more information about spellwork to help you learn about witchcraft more completely.