Observe, read and have the desire to learn about computers. That's what I did. I never read any books on computers, unless you call Maximum PC a book. Everything I learned was from the internet and various articles.
To tell you the truth, I think damn near everything I leanred was from Alvino at one time or another. Sure I picked up things here and there from other random people, but the vast majority is from Alvino.
is there somthing in particular you want to learn about? Go search google for forums on specific catagories. Like if you want to learn about overclocking go to xtremesystems.org. Not saying this isnt a good forum to learn stuff from just forums that focus on a particular catagory of computers go alot more in depth.