A) Using Linux (familiarizing yourself with basic commands) is one thing.
B) Learning about the Linux kernel (familiarizing yourself with the kernel's architecture) is yet another thing.
C) Leveraging Linux for a specific solution (for example, you might be using Linux for an embedded project) is yet a third thing.
1. If your main interest is "A)", Google for basic Linux tutorials. For example:
Don't buy a book (yet). Download a "live CD" first. Try it out. Experiment a bit.
Once you feel comfortable, *then* consider getting a book ....
2. If your main interest is "C)", however, then I'd suggest one of these two books:
1. Embedded Linux Primer, Christopher Hallinan
2. Building Embedded Linux Systems, Karim Yaghmour, et al