Safari books is a new tool for me. I have been paying for the expensive subscription ($45/month, unlimited use of the entire library) during my CCNA studies.
Ton of advantages with this. As long as I have an internet connection I have all of my textbooks with me without having to carry them. The site stores notes, bookmarks, highlights and such for each book. If something is giving me trouble I can search through multiple books looking for an alternative approach.
While I am not real happy with the price (did I mention I am paying $45/month!) I am getting a shit ton of value from it. I have decided to keep paying till I am done with the CCNA.
Oh, Safari books also lets you watch DVD's and such too.