So you want to learn Linux? The best way, the only really good way, is to get a distribution, load it on a computer and give it a try.
Any of the ones I mentioned recently at DesktopLinux.com in "Whats the best Linux for beginners?" will do fine.
However, you may need or want to read some materials about Linux to get up to speed. My favorite book is one Ive mentioned often: Robin "Roblimo" Millers book: Point and Click Linux.
Another good book is Dee-Ann LeBlancs Linux for Dummies, 7th Edition. Unlike Millers book, which focuses on how to use one distribution, this is more of a general guide to no fewer than seven distributions, including Ubuntu.
If Millers book is the blue plate special, then LeBlancs is a tasters selection of Linux delicacies.