You were asking about the reason I started. I was wanting to challenge myself to make a high quality app, so I was looking for ideas.
I spent my whole adult life as a professional musician with a popular band in NZ. When touring Europe one time, I met my wife, who was the singer in another band on the same circuit.
We both ended up leaving our bands and working together playing music. I moved to Switzerland, and found that I needed to learn some new languages!
I have a number of English speaking friends here, and one day one of them told me how he was reading Roger Federer's biography, and that he had bought it in both English and German to read at the same time.
That's when the lightbulb went off in my head, that I could make an app that makes this process easier. It's a great method to practice a new language, but keeping the two books in sync is a bit of a pain, and I realised that I could make this part of it easier within an app.
It turns out that it is actually quite complicated to program this, and there is a lot going on behind the scene to make it appear so simple and user friendly on the surface.
Currently all books in Parallel Books are free. This means that I can only use either texts that are public domain (copyright free) or that I acquire permission to use.
I'm also interested in finding very beginner level texts I can use, which have also proven allusive."
App store: https://itunes.apple.com/app/p
Facebook page (includes screen shots and demo video): http://www.facebook.com/parall