the in game books are a bit of a disappointment. when the game first came out I expected for them to be a bit like how the elder scrolls has their books but that's not the case. Almost all of them have one or two sentences and that's it. To get the lore you have to do a combination of everything, read the books, listen to NPC's, pay attention to the events (especially the chain events), do the renown hearts and actually pay attention to what they say, use the scouts, and also read what every clickable NPC tells you. It's quite a bit more work than most mmos to get the lore even though it's more believable to get the lore this way, just a bit more inconvenient.
Plus the personal stories and the story mode dungeons, they both add to the lore of course.