Yes, CentOS is your OS. CentOS is RHEL's clone. You can practice for your RHCSA and RHCE exam on that. Yes, RHCSA. RHCT is no longer there and they have changed it to RHCSA which I think is better than RHCT.
I would suggest RHCSA/RHCE Study guide by Michael Jang. It is pretty good and the explanation given by the author is awesome. Here is the link for the same: http://www.amazon.com/gp/product/007...T8YN2TFQ6EWKRD
Make sure (if you are purchasing) that you purchase 6th edition as 5th edition is on RHEL 5 which is no longer there. RHCSA and RHCE exams are now on RHEL 6
I hope this helps.