The Photo Guide bills itself as the most comprehensive guide to photography on the App Store and I am inclined to agree. This US$4.99 app is a 250 MB download stuffed with information on all aspects of digital photography. The guide covers cameras, from point-and-shoot to DSLRs, with plenty of info on the capabilities of the cameras built into each iOS device. There's information on filters, composition, HDR photography, lighting, you name it.
Aside from all that great information, The Photo Guide also has several nice features. For example, there are many sample photos that you can tap to enlarge and examine closely. The app keeps track of what you've read and what you haven't, and features a very comprehensive search function. As in any photo book, there are a lot of technical terms. Those are hyperlinked to definitions and explanations.
The app deals with cameras and shooting more than post processing. If The Photo Guide is missing anything, I would have liked to see some videos demonstrating various concepts in the "How-To" section.
While this guide is aimed at beginning photographers, there is much useful information for the more seasoned amateur. We've looked at other guides here at TUAW, such as Master your DSLR Camera which is $4.99 and the free Photography 101 that includes some nice videos.
I don't hesitate to recommend The Photo Guide to digital photographers. It took a lot of work to put this information together in one place. The photo examples are excellent, and the text is concise and clear.
The Photo Guide requires iOS 5 or greater and is optimized for the iPhone 5. It is a universal app, and works happily on most modern iOS devices.
Check the gallery for some screen shots.