Flow State: The Short & Sweet Book of Ecstatic Living: Your Complete Guide (in English) - Jetzt!, Freiheit ; Anders, Nino

It's quite short but very useful! The story helps you to understand the "feeling" of the flow state, and then the steps are simple enough to follow and try until you start to get there. I recommend a regular re-read, maybe once or twice a week until it's starting to become a natural process for you to follow before starting your study or work schedule.