This is a known area and OpenCV might well be involved, but still to start from the scratch.
How has something like Evernote's scannable app been developed. I mean, how does it automatically recognize a document using a camera and then extract it.
What are the UIKit frameworks involved here and what are the libraries that may have been used. Or any nice articles or blogs. How does one go about understanding this.
This tutorial is what you might be needing. Although, this tutorial is in Python but all these function are available in iOS bindings.
Here, are results you will get.
Once, you have the ROI i.e. the page, you should run OCR to detect the characters. For this you can use Tesseract and this tutorial might be helpful.
For anyone coming here now, there are better solutions now. CIDetector does precisely this. And to have it working on a live camera feed, you'd have to use it on live CIImages being generated by AVFoundation (rendered using Metal or OpenGL).
