Progressing from a novice hobbyist to an intermediate Linux user

This question hopefully gives novices a place to start with progressing with Linux.

I am an accompished Windows developer on the .NET stack. I know the network administration, registry and the services in place etc etc.

In other words, it's time to spread my wings and fly.

I have started using Ubuntu Linux and been bought a Raspberry Pi for Christmas. Beginner books always cover the history of Linux and then move onto file system navigation and apt-get usage, then they end.

What can I learn to take the next steps in learning the Linux platform. What do you see as mandatory knowledge for a prospective intermediate Linux administrator?