I don't think there is a clear answer. WOTC radically shifted how they made sourcebooks after 3.5 edition and they've deliberately shied away from being thorough at filling in the gaps in the lore since then. You may have to take whatever you can piece together from the adventure modules, online stuff, and the few novels they still print.
They figured out that they can sell books while only doing half the work by rebranding laziness and their inability to make a good plot as "DM freedom" and have their fanboys defend them.
