Uesp.net says there around 280 quests in the main game, so apparently there's still a lot out there!

Another multi volume series as long as Berenziah is The Song of Pelinal, although it's very different. It's about the Alessian rebellion and the fall of the Ayleids, which is a huge backdrop to the game, especially the Knights of the Nine quest line. It's written in a very ambiguous manner and it's hard to tell what's a metaphor and what's literal at times.