At around level 55 – 57 you should move on to the Kingdom of Sky zones. These zones are fairly small and you can travel on little clouds, so getting all over the zones goes fast. Leveling here is a piece of cake compared to a lot of other zones, there are tons of easy, fast quests that will level you up to level 65 in no time.
The first zone is the Tenebrous Tangle. This zone is especially good, because of all the repeatable quests in and around the mail island that you start on. These quests give awesome XP and can be repeated a set amount of times. The quests in Tenebrous Tangle should take you to at least level 60 before you move on to Barren Sky.