For those with a swimming background and/or access to pool-based or open-water swim coaching, this book is great. If you're not in one of those categories, this book will still be a great source of info, but you'd need to supplement it with additional material and/or expertise from other sources.
That's because the book is a little thin in its discussion of how how a swimmer can put together a sensible, interesting training programme. The book presents a few sample sets that people may swim, taken from training sessions performed by world-class swimmers, and a few sets for specific purposes. It also outlines a very rough 12-month programme for a 25K-marathon swim, showing a suggested mileage and time spent in open water, acclimatising to the cold. That's all useful, but information suggesting how the intermediate swimmer would bring some periodisation into the preparation for a season, what a typical week might look like, and how the self-coached swimmer would gradually increase distance or intensity over a series of weeks and months, is not provided.
In summary, I share with various other reviewers the opinion that this is a valuable book. However, swimmers would do well to pick an additional book with suggestions for training progams, or indeed they'd need access to a coach, join a swim team, or already have a solid understanding of how you prepare for various distances.