During my hack-and-slash sessions, I encountered some lore tomes that seem to be a part of a larger set. I have encountered three sets so far, Lian's, Deckard Cain's and Lachnadan's. Is there some sort of reward for "collecting them all"? I assume there would be some achievement and maybe banner widgets, but maybe there's something more to that?
There is no real reward such as a useful item. However, you do get various achievements and some XP for each book.
The meta achievement is Grand Lorekeeper of Sanctuary
Here is the full list of "read lore" achievements: