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Google Maps was nothing short of revolutionary when it was released. It was the "perfect storm" of technology (AJAX, Web Services) and paradigm shifts (Web 2.0). You may have seen mapping websites before, but nothing raised the bar from "web page" to "web application" like Google Maps.
Part of what makes Google Maps unique is that you aren't limited to running it just on Google's website. They released a free API that allows you to embed Google Maps in your own web pages.
Google's tutorial is a good place to start, but at roughly a dozen pages, it offers little more than a cursory overview of the API. The Pragmatic Programmers' Google Maps API offers over 60 pages of detail: fresh, independent insight into the strengths and weaknesses of the API coupled with lots of demo code and real applications.