FREE eBooks in Apple iTunes / iBooks..... Hundreds of them.

[#1]

Just go to iTunes, iTunes Store, Books (At top of screen)
Select Free Books...
(Careful, some books are chargeable)
Various available for iPad, Mac OSX devices

The free eBooks may be a short term offer.

Hundreds there !
Once downloaded, or even whilst downloading, you can see them in iBooks.
Yes, you need iBooks, which is included in Mavericks. Mine anyway !

[#2]

Is it possible to get iBooks for older Mac versions?

[#3]

I don't think that is possible
http://www.apple.com/uk/ibooks/
You may be able to free upgrade software to Mavericks, depending on your current OS, and Mac