Rook used to be an awesome tank, before his armor got slashed. Now, everyone stills wants to pick on the giant lumbering tower, but he can't take it. Because he's such a ginormous target, I think it could be beneficial to run him as a Tank. Bear with me now, and yell at me for my horrible ideas...

Early game, focus on Tower and Castle upgrades, especially Structural transfer. Get God Strength as soon as viable, and you can pretty much ignore the slam. You're main goal is to get lots of assists, kills, and experience, in order to afford the items you'll need to tank. I don't like giving an exact build, because every game is different, and adjustments need to be made. Here's the item list I think you'll need.

Favor- Dark Crimson Vial. Useful for getting out of tough spots

Breastplate - Armor of Vengence or Groffling Warplate, depending on funds and playstyle

Hat - Theurgists Cap. If everyone is attacking you, this can be useful for your flanking teammates.

Boots - Ironwalkers or Desperate boots. The previous is a nice 1000 armor, the latter helps you stay alive long enough to escape, if need be.

Gloves - Slayer's Wraps. Minion damage and Cleaving attacks are irrelevant, Rook automatically Cleaves after a certain point.

Other- Always have a teleport scroll or two, to get out if you really need to.

I also like to pack an orb of defiance, it can be rather beneficial.

The All Father's Ring is a nice addition to any build.

Last, Bullwark of Ages. Never tank without it.

Okay, so if you get all of this and do the math, you end up with +5550 armor, which translates to around 65% damage off from melee attacks, and the Bulwark gives you 25% off ability attacks, I think. Coupled with the orb and all the added health benefits, you should last forever, or close enough. When you get low, Dark Crimson Vial and Structural Transfer should be a nice pick me up. You should be doing around 500 weapon damage, give or take, and everyone will want to shoot down the big walking tower, because who wouldn't?