I don't know if it's still in print, but "Rune Magic" by Nigel Pennick is a decent introduction into norse type magic(with runes!). I had a great book that was an intro to druid magic, I'll look around and see if I can find the title. If you're interested in learning more about Wiccan type magic(magik, magick), you might consider googling for covens in your area. I can almost guarantee that there is one somewhere nearby that would be willing to teach you. You can play it however you want (telling them you're a writer, or telling them you're interested in learning magic) but they're usually pretty open. In my experience most of the Wiccan books out there don't actually have any of the mechanics of magic, they have specific spells. And while reading enough of them might be able to give you a solid background, wading through some of that material can get old fast.
I'm kind of like you, I like reading about mythology and magic and religions. There are MANY magic systems out there, and MANY beliefs in that area. If you really want to use something in your writing, I would focus on a certain area, or simply get an overview of all of them and make it up for yourself. If you want to "borrow" a specific system let me know and I probably have more book recommendations for you.