One of the classic stories of children's fantasy. I read it when I was young, and at WorldCon, this year, I was happy to hear a panelist refer to Assistant Pig Keepers, with no other explanation necessary. Everyone knows this story, and rightly so. If you don't, read it now.
There's no one thing that's particularly magical about The Book of Three. The writing is smooth and evocative, the setting is credible, the characters are human, and the humor is evident. Alexander has done a terrific job of balancing it all out, with a tone that's perfect for young readers. Taran is young, he sounds young, and he acts young - all in very believable ways. The attitudes are a bit dated, and some of the scenes are a bit thin, but the story still works well.
If you're new to the book, you'll have seen all this done many times. In fact, when Alexander did it, the elements weren't particularly new. But few have done it so nicely. Recommended, especially for younger readers - nothing too frightening, and some good lessons.