Examining the official VMF files is the best way to learn how to create maps for Source SDK games.

With each Authoring tools Valve releases some example maps VMF files. E.g. for Alien Swarm there are 3 maps available. For L4D 2 there are also 3 maps and a bunch of tutorials. For a Source SDK there only is a small 1 room example.

Why not release sources for all maps for each game? Doing this would allow people to see more to know more and create more high quality addons and mods.

For now the only way to see the source of a map is to use some of decompilation tools like BSPSource. But they corrupt something that can lead to misunderstanding of techiques.