Having written three books on programming in a series that was called "By Example," I know first-hand that sometimes several hundred pages of written text doesn't really beat the ability to see something for yourself, tweak it, and find out what happens.
Not that Google has ever really been that big on documentation anyway; but this morning, it's unveiled something that's perhaps several hundred times better: Its new AJAX API playground lets JavaScript programmers not only sample all the major API calls in Google's toolbox in the context of functions, but tweak those samples and see the results live.