Check out Gruitale.com - they have some easy-to-read intros on the theory & botanicals needed. Getting a hold of them is a little trickier.
wildweeds.com has pretty much everything you need to start, and harvest time is just over, so it's a good time to experiment. Alas, I'm currently between breweries (I usually work out of houses of reasonably hygenic bachelor friends), so haven't made a brew in a while.
I've heard really good things about yarrow & mugwort beers though; if you can get through the initial very-bitter taste, they have a helluva kick.
John