D.P. Walker, Spiritual and Demonic Magic (more for the Renaissance approach)
These books might give you a few ideas as well:
Owen Davies, Grimoires (a bit light on thesis, but absolutely full of interesting trivia)
Ioan Couliano, Eros and Magic in the Renaissance (his conclusions are debatable, but his argument is intriguing)
Frances Yates, Giordano Bruno and the Hermetic Tradition (very influential, although her thesis rests on extremely shaky foundations)
(I can provide plenty more for anyone who's interested. If you're going to pick one to start with, get either of Kieckhefer's books.)
In terms of online resources, Twilit Grotto is probably best, though I see it hasn't been updated recently. The Alchemy Web Site is also quite handy, if you're fond of that sort of thing.