Honestly, I'm surprised no one has mentioned 'The Art of Learning' by Josh Waitzkin yet. It's not branded as a 'flow' book per se, but it's the single most practical guide I've ever read for entering that zone. Waitzkin, a chess prodigy and martial arts champion, breaks down the psychology of incremental mastery in a way that just clicks. The chapters on making smaller circles and building your numbers are pure gold for creative work—it's about deepening focus in tiny, manageable layers until the outside world drops away.
Csikszentmihalyi's 'Flow' is the foundational text, obviously, and it's brilliant for understanding the theory. But for actually applying it to creative output, Waitzkin's book takes the cake. It taught me more about building a sustainable creative practice than any 'creativity' guide ever has. My painting output doubled after applying his principles, not from forcing it, but from learning how to structure my entry into the work.