Your strategy depends on the map. One of your top priorities is to blunt their push, and that means buying your citadel upgrades early on. Take advantage of their lumbering move speed and grab flags from under their noses. On some maps, particularly Crucible, if a Rook can get through your front line of towers early enough the game may be over right then and there, so ensure you have the capacity to blunt that advance.
Remember that Rook can only support two towers when he's under level 4, and this isn't enough to stop you from running up and attacking him. Because Rook has lots of hit points but little armour, he can't rely on passive hit point regeneration and needs to return to base to heal up. Don't let Rooks get an early game foothold, and you'll be in a good position to turn things around mid-game.
Above all else, don't allow a Rook team to control flags. Rook has to lumber out to go and get them, and if you haven't done any dying he's going to be at a significant disadvantage so far from his tower farm. At very least it'll take him quite a while to get there and back, by which point his original tower farm can be damaged significantly. Do everything in your power to stop a Rook advance, but be careful not to die. A good Rook player gets his kills from his enemies being foolish and straying into his tower farm; don't become first blood, because once they start getting gold a Rook push can be very hard to stop.
