For previous very relevant stories, as everyone else reccomended: Armor Wars and Demon in a Bottle. They are also very good ones and help define much of the character.
For modern IM jumping point i believe Extremis is essential, not only is the current Iron Man tech explained but also is his origin restated (timeslipped of course) but also his current mindset is explored (interview + talk with company board).
After that you can jump to Invincible Iron Man.
Of course, the issues and arcs coming after Extremis are, imho, quite good so well worth the read.
So basically: Armor Wars, Demon in a Bottle, Extremis (+following arcs for fun but not essential), Invincible Iron Man for current ongoing.