This is the key text on what it means to be in flow, how flow is created and maintained.

Amazon say:

What really makes people glad to be alive? What are the inner experiences that make life worthwhile? For more than two decades Mihaly Csikszentmihalyi studied those states in which people report feelings of concentration and deep enjoyment. His studies revealed that what makes experience genuinely satisfying is 'flow' - a state of concentration so focused that it amounts to complete absorption in an activity and results in the achievement of a perfect state of happiness. Flow has become the classic work on happiness and a major contribution to contemporary psychology. It examines such timeless issues as the challenge of lifelong learning; family relationships; art, sport and sex as 'flow'; the pain of loneliness; optimal use of free time; and how to make our lives meaningful.