If you’re a 4th Edition Dungeons & Dragons player then the two Essentials books that you most want to look at are Heroes of the Fallen Lands and Heroes of the Forgotten Kingdoms. Each book is presented in a similar style to a stand alone Player’s Handbook with Heroes of the Fallen Lands introducing new builds for the classic D&D classes (Clerics, Fighters, Rogues, and Wizards) while Heroes of the Forgotten Kingdoms introduces new builds for Druids, Paladins, Rangers, and Warlocks. Each of these books stands on its own perfectly well and you don’t need to buy both if you’re only interested in the classes presented in one of them.