Welcome to Macintosh!
I don't know of any free books, sorry.
For the most basic stuff, you could go to Apple's website, and look for an electronic copy of the booklet that came with your iBook when it was new. Very basic.
I'm sure others will have better suggestions - I just wanted to welcome you aboard.
This site is a wealth of information - and a fair amount of cr@p thrown in for good measure. Try doing a search in the forums - the button is just up above a bit, about 2 inches from the top of the page, and 3 inches from the right.