I would recommend this book: Deep Work by Cal Newport. If you can, listen to it (which you can do free when you first download Audible).
For lots of depth, look here.
Tim Urban, from Wait But Why, did a fantastic TED talk too right here.
Here: why hard work makes us the happiest (from Mr. Money Mustache).
This site lets you repeat any song in the world for free. I find listening to songs on repeat helps me focus very well.
I have no apps to recommend as I find digitizing tasks only makes them harder to manage.