Nokia Camera is a good place to start if you want to learn manual controls and their effects on your photos, but now I'm finding myself using Proshot more because of the advanced features
Pro shot on my 930, and use Nokia camera and Pro Shot on my 1020. The feature set of pro shot is closer to using my dslr though just a pity it can't be selected from the camera button.
in the Store, there's a special category called "photo" for that kind of apps...
scroll to the "best rated" sub-category and you'll find some good photography apps there...