What they’re doing is a series of painted images in movie clips (some with 2D animation over top) that are fit together and continually scaling. Basically you take one image, take a small area at the center of it, and create a new image with more detail from that small area. The borders of the new image match up with those of the larger, outer image. If you look closely you can see that when you approach the newer image its detail is finer than its surroundings but they blend pretty well. I’m guessing they’re just fading out the edges of the new incoming image so that any inconsistencies in position and quality aren’t as noticeable.
You could actually do something like this on the timeline in Flash but I’m sure this site is doing it somewhat more programatically. They’re likely doing some culling to remove images that have since zoomed out of view and only add in the upcoming images to zoom in when they’re needed. You’d have to do this because having an large number of movie clips with rather large bitmaps all scaling siultaneously would very much tax some computers and make the whole thing very choppy. Done properly, you would only ever have between 3 to 5 images showing depending on the size of the screen you’re displaying it in.
I can’t really recommend any reading material for this but I think you could probably do it without too much difficulty depending on your level of ability with Flash. The tough/time consuming part is getting all those really nice images that all fit seamlessly together.