You can tell this was a PhD thesis based on interviews with established authors, then reworked into a manual for interested writers. But that isn't a bad thing. This book really influenced the way I understand writing as a first-person activity, and also the way I practice and critique it. I knew I went into flow states, though I didn't know they were called that - I'm yet another example of someone who writes partly to chase that trance - but I didn't feel I had any control over it. After this book, writing has became a habit I try not to overdo at the expense of other demands, rather than what it was before, something I tried to get around to and often enough did.
This is by far the best source of information on the topic that I know of, likely as much as you'll want to know and more. It demystifies the factors that lead to or impede flow, as reported by many successful writers - some of whom experience flow often, others never.
It's clearly written in a pleasant style. This is not an abstruse or overtly academic book, despite its origins. I'd call it gently academic. It's more practical; the advice can be applied by anyone. Takeaways from the different interviews are presented in sequence, and the chapters are full of snippets from the respective authors. But the audience is perhaps narrow, let's say select. If the idea of being fully absorbed to the point of lost in the act of writing is interesting to you, and you don't mind an easygoing but researchy/interviewy read, you might find it completely fascinating.
I was already writing relatively often because I liked being in flow. For me the first time I remember was in 4th grade, when I was writing a story for an assignment. After a bit of thinking about the setting and basic plot - some effort to get started - I found that I was lost in the story I was reading, forgetting where I was, all that stuff I enjoyed about reading a good book - only I was writing it. If you want to discover that experience, I can't say whether this book would help someone who doesn't know what it's like, at least while writing. But reading this might work well, as the tips are based on patterns found in the data. Like most of psychology, flow feels like magic but isn't actually magic. There are parameters, metaphorical knobs and dials. If that kind of thing, thinking at all mechanistically about experience, puts you off - if understanding the biology of a flower degrades your experience of the beauty of the flower - you might not like this book. That isn't how I am (the flower often gets more beautiful) so I loved it. But the book is undeniably informative.
A key fact came out of the research: being in flow does not correlate with writing better. You may feel that what you wrote all at once, easily and fluently, is better than what you chiseled together, painfully, against the current and the tide and the prevailing winds and all the other demands in life - but that isn't how it works. Writers who experience flow are not more or less capable than writers who don't. The same goes for writing sessions in which flow was present or absent: flow doesn't tell you about whether the result is good or bad (or even whether it reads smoothly). However, flow does correlate with enjoying writing, and writing more. So there are more professional writers who go into flow than who don't - people who find "the zone" enjoy writing, write more, and get better.
Flow is something you can influence - hence the value of this book. But note that Writing in Flow is specifically about the feeling of flow while writing, rather than a reader's impression that the writing flows well. Two different kinds of flow! Sometimes they do seem to align, other times definitely not. Just removing that preconception - "It flowed when I wrote it, so it must flow for a reader" - is helpful.
I do believe there is something to the words that come naturally, and this is why redrafting often works better than just tinkering in a word processor. But that isn't necessarily a question of psychological flow states. I'm guilty of almost never redrafting - I tinker to hell, but I know it's an issue. Most writers swear by reading out loud, which gives your ear a snapshot of the language fluency, and starting at the top and writing the thing over. Again, that's a concern closer to the other kind of flow - the reader's. You don't have to be in a trance to put the reader in a trance, and if you are, it doesn't necessarily translate.
Earlier I had this rated at 4 stars because it eventually gets less exciting, like any news article or journalism piece with an inverted pyramid structure (most important info first). But on reflection, that's quite all right, and I think the many exciting bits are far more representative, and ultimately a book that strongly influenced how I see and live deserves the hat-tip.