So the first of the essentials is not essential?
It's the rules for D&D, from levels 1-30. This includes (IIRC) character creation, combat, terrain, skill challenges, etc. It does not include the actual races, classes, magic items, rituals, adventuring and encounter-building stuff. These will be in the two "Heroes" books and the DM Kit. The monsters, of course, will be in Monster Vault.So what is the 'Rules Compendium'?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.