How often did you meet in your practice Rook endings? I claim that more than half of all games pass through this part! So for improving your chess skills you'd study rook endings extremely carefully.
The rook is a powerful long-range piece and should generally be used actively. It is not adept in passive defence and the blockade of enemy passed pawns. For this reason the game may still be unclear even when one side has sacrifice his rook!
Rook ending consist in three topics:
1) Rook vs Pawns
2) Rook vs Rook
3) Double Rook endings.
Start from the first Rook vs Pawns.