The short answer is that there isn't a way to read another app's files unless they are in a compatible format (txt,csv,etc). It sounds like you're wanting to OCR a screen capture.
This might give you something to work from: OCR a screen capture
Some apps (like iTunes) save their library files in some proprietary XML format. The file may not end in ".xml", but it's still a XML file, in which case you can parse it to get what you need.