Many essential classes (and subsequent classes like the berserker which is more post-essentials) are written in such a way to imply that you should chose from the listed powers, but they allow for you to actually take from any class power from any book (see Heroes of the Fallen Lands, page 57 "Other sources" sidebar for RAW statement of this). A berserker choosing PHB2 barbarian powers is legal RAW, but loses out because it can't rage and will have half (or all...) its at-will throw it into a fury as they are primal, likewise PHB2 barbarian can take berserker powers, but loses out because it can't Fury (this effects the at-wills the most).