Yes it's great that the iPhone gives you a decent still camera in your pocket. And sure, the seemingly endless array of photo apps makes the possibilities limitless. However, humans tend to do better at tasks that are littered with limitless possibilities. Allan Hoffman's book:Create Great iPhone Photos: Apps, Tips, Tricks, and Effects "Create Great iPhone iPhotos" brings the breadth of iPhone photo options down to a manageable scale.
The first chapter reads like the manual Apple would supply for the iPhone camera if Apple still made manuals. It is a sensible, thorough overview of the camera capabilities (and limits) of the iPhone camera and the Apple-supplied Camera app.
The bulk of the book goes on to serve as sort of a series of mini-manuals for some of the most popular iPhone photo apps. Unfortunately, a few of the apps are already gone from the store. Such is life lived at app speed. These chapters serve as a great way to discover app you may not have already been familiar with, and to eliminate some of the trial and error of purchasing photo apps only to be disappointed.
There's a highly enjoyable (and unfortunately, quite short) section of interviews with some of the most successful iPhone photographers.
The focus on apps really sets this books apart, and can bring you newfound joy to using your iPhone camera, whether you're just getting started, or consider yourself an old pro.