The Plane of Tranquility (requires min level 46 to enter via the teleport stone in The Plane of Knowledge). In the first room you zone into you will see a stone table with a floating book above it. This is a locked object and can be picked repeatedly to skill up with no minimum skill requirement.
If you are below level 46, or on a progression server alternately you can go to Befallen or the elevator in Paineel are also good locations.