I've not bought or read this particular book, but it specialises in rook endgames.
http://www.everymanchess.com/display.php?id=261
I have bought other books from this site and they've been very good.
another good book is Dvoretsky endgame manual which i do have and this covers all endgames including rooks. An article by him can be found at this link
http://www.chesscafe.com/dvoretsky/dvoretsky.htm
He explains some positions about rook endings that even grandmasters get wrong.
as for free books or online courses i'm not sure.
Can anyone point me to a good book or online article that teaches and gives examples of optimally using your rooks? I've noticed that I have issues with my rooks that stem from the early middlegame all the way through the endgame.
Thanks in advance,
Chessbuzz