Hehe, I think I was lvl. 19 too JUST before I battled Laiken and claimed MY battletower. There isn't too much to do on Sentinal Island though, and don't bother trying to find this Spellbook high up in some strange rock/tree. You can pick it up later!
Regarding the magic spell book in the ship, its better to use it yourself than turning it in for the quest, as you gain 2 skill points by using it, where turning in for the quest only provide you with few k exp only.
