Hi! I just joined here this morning so I hope I'm doing this right...
I LOVE applique! Some of my favorite books are just about anything by Elly Sienkiewicz, but good luck finding some of her older titles at anything resembling reasonable prices. Good grief, I've seen a few of her titles selling for well over $100 each on the used book sites (Baltimore Beauties and Beyond, Volume 2, comes immediately to mind). Elly's original series is a wonderful resource that details numerous different techniques and styles of applique.
Another great little book that helped me a lot when I was just beginning is Ami Simms' Invisible Applique. It was that book and her technique that finally caused the applique light in my brain (and fingers!) to switch on.
As was mentioned above, the Rodale books are also very good, with lots of very clear illustrations. I don't think you could go wrong with those.