Even though there are other good introductory books, and more modern ones (particularly Janice Kim's "Learn to Play Go" series), this one is still a classic every beginner and intermediate player up to 20 Kyu should read. Iwamoto covers a lot more material than other beginner's books, so you might not understand all on the first reading, but it's definitely worth to read it twice.
At half the price of "Learn to Play Go" Vol. I it covers about twice as much. Plus, at half the size it fits into every pocket.
I own "Go for Beginners" and "Learn to Play Go" Vol. II + III and I wouldn't want to miss any of them. If possible, read them all.