Storm of Iron is a great book for a newcommer to 40k. Fall of Damnos showed you what the good guys are like, Storm of Iron will show you the bad guys. It's one of McNiels better 40k era books.
After that it's a question of where your interests lie.
If you like the Imperial Guard, normal humans standing up to the horrors of the galaxy then i would recommend Cadian Blood, Gunheads, Double Eagle and Ciaphas Cain. Each offers something different; Cadian Blood features infantry against the forces of chaos, gunheads is about tankers agaisnt orks, Double Eagle fighter pilots and Ciaphas Cain about a commissar and is a more light hearted and comedic look at 40k. You may also look in Gaunts Ghosts, a long running series of books. I personally dislike them, finding the protagonists far too 'mary sueish'.
If you like Space Marines than Helsreach is a great read, ADB really nails the Astartes mindset. Most Space Marine are 'bolter-porn' unfortunately, meaningless battle scene after battle scene.
If you want fallen heroes, the chaos space marines then you can do no better than ADB's Night Lord's trilogy (Soul Hunter, Blood Reaver, Void Stalker). They are truly some of (if not the) finest 40k books written. Not only do they posses evocative action scenes but actual character development. Proper character development in a 40k book! ADB also really nails the 40k setting and atmosphere.
If space ships are your thing there's Execution Hour and Shadow Point by Gordon Rennie, two books detailing a Captain of the Imperial Navy's experiences in the Gothic War.
If you like giant warmachines then look into Titanicus by Dan Abnett.
One of the seminal pieces of 40k 'literature' would be the Eisenhorn series again by Abnett. It follows the Inquisitor Eisenhorn in his investigation and descent into radicalisation. It's a great read again featuring that elusive character development and is far more than just action scenes, giving you a taste of life in the 41st millenium.
If you'd like to know how 40k became 40k then pick up the (in progress) Horus Heresy series. Some of these have featured on the New York Times best sellers list and they're generally a cut above the standard Black Library fare.
In terms of authers Dan Abnett and Aaron Dembski Bowden are popularly recognised as the two best (Aaron exploding onto the scene). Graham McNiel is a common third.