Man, I always see people asking this, and the answer's kinda messy. Those 'Diablo' novels by various authors – like 'The Sin War' trilogy or 'Legacy of Blood' – are absolutely set in the same universe and directly feed into the games' lore. They're not just side stories; they're the foundation. Reading 'The Sin War' completely reframed how I saw Lilith and Inarius in 'Diablo IV'. That whole schism, the creation of Sanctuary? That's all novel territory first. You get Uldyssian's story laying the groundwork for the Nephalem power scale we see later. The problem is Blizzard's never been great at signposting the essential reading order, so it feels like a treasure hunt through used bookstores or digital archives.
That said, calling them 'connected' undersells it. They are the storyline, just in a different medium. The novels flesh out Deckard Cain's younger years, the Horadric Order's internal politics, and the geopolitical mess of kingdoms like Kehjistan long before the games show you the ruined versions. They make the world feel lived-in, not just a monster-filled loot piñata. If you only play the games, you're getting the cliff notes version of a much denser history. The character motivations, especially for figures like Mephisto's manipulations or Rathma's neutrality, have so much more texture in the books. It's a shame they aren't integrated more clearly, because they massively enhance the experience.