This book comprehensively covers the game of Go, and justifies the word "Beginners" in the title by at least starting with the basics. But sadly I wouldn't recommend it as a book suitable for someone completely new to learning the game. Iwamoto's knowledge is impressive, and his writing style clear and descriptive. But what he offers is more a demonstration of his own Go understanding, rather than explanations to help others understand.
The style is concise (but not terse), and moves rapidly from the basics through increasingly complex board positions and plays. But too often he dismisses a new layout of stones (just a jumble of black and white to the novice!) with statements such as "in this position, black's best move is obvious..." It may well be obvious to an expert, but the whole point of the book ought to be to explain why a certain move is the correct/best one - and the lack of this explanation makes the book frustrating and ultimately unsuitable for beginners. The style is also occasionally patronising - Iwamoto refers to one common beginner's error as "boorish behaviour" which is neither helpful nor encouraging to a novice looking to an expert for advice and motivation.
This book's strength however is its comprehensive coverage in a neat and small package. I think it works much better as an 'aide memoire' for the developing player, who is past the novice stage and wants some reminders of strategy and positional play to bolster learning gained elsewhere. Approach this book with that aim and it succeeds wonderfully. Approach it as a complete beginner wanting a gentle, hand-holding introduction, and you may be confused and disappointed.
(Personally, I found 'Teach Yourself Go' by Charles Matthews much more suitable for a complete beginner to the game).