Magic Forums

I'm a new poster but I've looked through a lot of the threads and I haven't seen a topic like this before, I've got a couple of books on witchcraft, several with extensive lists of herbs and their properties, but none with a table or any explanation as to WHY certain herbs are believed to have certain properties.
Personally, my belief in magic comes from a deep seated feeling that certain things are solidly true. Red is hot and ignites passion, blue is cooling and calming, meditating on an outcome can make it more likely to happen, these things are not only believed by witches but also have a solid foundation in psychology and science. I tend to disregard things that have been made up by modern people to fill in the gaps of what has been lost from the collective memory since Christianity.
I want to know things like, why, exactly is eucalyptus supposed to be good for abundance spells, why is sage supposed to be good for clearing negative energies? I don't want to be using a list, made by someone I don't know, of herbs and their properties with no evidence (even lore from quotable sources) as to the legitimacy of these claims. I think it's important to understand fully why you're using something in a spell, otherwise your intent can get scattered.
If anyone can recommend me a good website or book on this, (preferably a book available in the public domain or a small book I can get for, like, a fiver) I would really appreciate it.