Linux From Scratch
From the days when installing a Linux distribution required a 300-page bible, to the days of Live CDs, the question "How do I learn Linux?" doesn't seem to have a ready answer. That's probably because the question itself is a little vague. If you want to learn how to use a Linux distribution, trying out one should help you. If as a network or system administrator you want to learn how to manage one, the SAG and NAG guides should get you started. But if you want to learn how a Linux distribution works, how it interacts with all the pieces of free software internally, then you need to lock yourself in a room for a weekend with a copy of Linux From Scratch (http://www.linuxfromscratch.org).
While traditionally LFS has required an existing Linux distribution to borrow several necessary files and libraries from, the project also has a Live CD (http://www.linuxfromscratch.org/livecd/index.html) that includes all the required files and the LFS manual as well.
The project has come a long way since its first release in 1999. It has several developers and users and various sub-projects. In an email interview I ask Gerard Beekmans, the guy who couldn't learn enough, how his little "note keeping" project has grown over the years.