Well, as you know, the "red box" come out on September 7th, though it's not actually called the red box, but rather the starter set.
The next essentials books are the Rules Compendium and Player's Essentials: Heroes of the Fallen Lands, which both come out on September 21st. I'm assuming that the Player's Essentials don't require the Starter Set, so that "seasoned 4e players" can skip the red box and get the other books. However, this I'm not sure about, and if anyone knows for sure (having seen the red box themselves, or having heard something from a reliable source), I would like to know.