It depends on which aspect you wish to delve into. Shattered Isles encompasses a range of topics and lore, such as Pirates, Cults, Ferumbras, the stories of the NPC’s from Liberty Bay and even ancient Elven civilizations.
If you're keen on the Pirate lore, I suggest starting with the books scattered across the islands. Moreover, there are numerous quests, such as the Pirate Outfit Quest or Treasure Island Quest, which offers a deeper understanding of its story.
The cults’ lore is very interesting too. However, for a comprehensive understanding, it's essential to explore other regions where they arise, like Ice Islands or Ankrahmun. Essentially, all these cults have a purpose and a leader, which ( in my opinion), is closely linked to the story of Ferumbras.
Regarding the stories of the NPC’s from Liberty Bay, I recommend talking to the NPC’s and asking about their opinion about the city. They will provide insights into the tensions and political conflicts between the native inhabitants and the colonizers. To my knowledge, this narrative directly correlates with the rebellion of Thais.
Concerning the ancient Elven civilizations, the NPC Malunga can offer some information about this topic. Nevertheless, similar to the cults, I suggest broadening your knowledge by visiting other cities and reading some books for a more complete perspective.
In the end, you'll discover how these narratives are inherently interconnected and how they unfold in particular contexts for specific reasons. From this point, you'll be able to formulate your own conclusions and theories. I hope it helps, best regards.