2920: The Last Year of the First Era
Probably the longest story in the whole set of in-game books. I guess it could be called a novella.
I read that one first, without any prior knowledge of ancient Elder Scrolls Lore (just the events of the Oblivion and Skyrim campaigns). It was a little tough to get into at first because it just keeps switching perspective between a bunch of different characters who don't seem to have any connection between them. But the story slowly builds up and eventually it all comes together, and by the end just about everyone's been betrayed or murdered.
I'd recommend reading it twice because the second time around you'll have a better idea who's who.
Also, apparently some of the main characters like Vivec and Almalexia had roles in Morrowind, which I haven't played, so maybe it would be even better for someone who's played through Morrowind.