I can heartily recommend "Backyard Medicine - Harvest and Make Your Own Herbal Remedies", (by Julie Bruton-Seal and Matthew Seal). It's an excellent book. I have lots of experience with both growing and preparation of herbs and still learned plenty. There's some very interesting history and folklore included in each entry and the photography is an adavantage if you are just starting out.
As you learn more, you'll see that many of the most valuable medicinal plants are NOT all that rare, in fact some of them grow in your back
yard, on old homesteads, and the like. Many are considered weeds, or (gasp!) invasives...speaking of which, I just read "Invasive Plant Medicine- The Ecological Benefits and Healing Abilities of Invasives", a really fascinating look at what plants do for the environment, and the body. It's by Timothy Lee Scott, with a great foreward by Stephen Harrod Buhner.
Look, also, for anything by Susun Weed, not much on cultivation, but lots of sensible advice on usage.