Amazon has three pretty powerful things going for them, and two are entirely their own doing.
Number one: Amazon is, by far, the most book-industry-focused company that is actually active in endeavors much larger than the book business.
Number two: Amazon executes.
Number three: Amazon is the runaway market leader in the only two segments of the book business that are growing — ebooks and the online purchasing of print — and they are cleverly leveraging the leadership position they have to make challenging them even more difficult in the future.
Details on the three points