iBooks is not available for Macs, however, you can download a free Kindle Application from Amazon, and purchase books through the Amazon Kindle store. If you do ever get an i-device, you will be able to download the app to those devices as well, and get any books you have purchased or are currently reading synced up to them.
Go to the Apps Store on your Mac, under Categories do a search for Kindle. Download, then set up an Amazon account on Amazon.com, go to the Manage My Kindle under your sign on ID and set up you computer as a Kindle authorized machine. Once that is all done, go to the Kindle store and purchase your books! (lots of free ones too....)
Detailed help for the Amazon/Kindle side of all of this is available under Help on the Amazon website.