Through the characters of Gus, Bill Bob, Titus, H2O, and Ma he is able to convey a philosophy and way of viewing our world that is a slap in the face without sounding pompous or preachy.
The book can be read as a comedy in and of itself but interwoven through Duncan's creative and entertaining characters is a philosophical masterpiece that has hints of Desert Solitaire, Zen and the Art of Motorcycle Maintenance, and Coyote Blue.
Second, the plot isn't about fishing, or living in harmony with nature; it's about a young man's discovering what life really is. The Perfect Schedule - young Gus's plan for getting in the absolute maximum number of hours a day fishing - turns out to be a horrible failure. It takes a long time for Gus to realize something is wrong, including a harrowing adventure with a drowned man and some pretty serous sickness. Now it may be - ahem - that fisherpersons are more stubborn or more stupid, but Duncan has Gus discover that there are things more important than fishing, and that those things can lead to still greater things. And that all of that can make the fishing better.
Third, while Duncan and Gus poke immense amounts of fun at it, this really is a re-casting of Izaak Walton's _The Compleat Fisherman_, although Walton is nearly unreadable and Duncan writes extraordinarily well. This book is also about more or less the same thing as those "witlesses" that Ma brings to grief, although both Gus and the Witlesses would likely deny it. One of Duncan's subtle messages is there, too.
Fourth and last, like a fish taking a fly, when you read this book you will be so dazzled by the gorgeous fly of Duncan's humor, writing and characterization that you will miss the hook and line of his real message until, like Gus, the line of light has you and you feel that gentle tug in your heart.
Beautiful and subtle, hilarious and passionate, charming and amazing, this book is simply an astonishing piece of writing. It's one of my ten or so favorite books, and likely will be one of yours, too.