The North (boxed set) - free on the WoTC website
The Savage Frontier - 1st edition supplement, revised into the North
Netheril - Empire of Magic - (good background on the history of the area)
Waterdeep and the North - 1st edition, but good
Dwarves Deep - prime supplement on Dwarves of the realms
Silver Marches - Good general area stuff
Demihuman Deities - great book, as well as the other 2. Monster Mythology covers the non humanoid faiths.
Volo's guide to the North - a good read, covers a lot we shall look at
Lost Empires of Faerun - another good look at the history of the area
Here is a list of some locations you might need to know or have some familiarity with, though most characters will know little.
The Nighthunt Inn
The High Forest
(these are just a few of the places you might want to look into or investigate)
(Here is an excerpt from Volo's guide to the North about the Delimbiyr Vale area)
...From Volo's Guide to The North
Often forgotten by many are the easternmost lands of the North. Overland travelers are the exception.
These folk often stagger into Llorkh or Loudwater more dead than alive, heartily glad that people dwell in Delimbiyr Vale!
The term Delimbiyr Vale is always used to refer to the upper reaches of the River Delimbiyr (also known as the River Shining) and its tributaries. The Vale reaches from the westernmost fringes of South Wood up the river valley to its headwaters in the Nether Mountains. The High Forest is not considered part of the Vale.
The traveler may sometimes hear about the Upvale and the Greyvale. The first term refers to the open grasslands between Tall Trees and the Far Forests, now controlled by Hellgate Keep. The Upvale used to be a series of pastures and tilled farms held by humans, despite almost annual orc raids.
The Greyvale consists of the grasslands drained by the Greyflow and the Loagrann, the three-branched river that joins the Greyflow at Orlbar, northwest of Llorkh.
Old maps also name the three headwater streams of the Delimbiyr, from west to east, as the River Aulantrar (or Deepingstream), the River Starsilver (or Starsilver Stream), and the Norlnryn.
Travel in the Vale is dangerous. One must beware of bugbear raids, increasing forays from Hellgate Keep, and the tightening grip of the Zhentarim. Because of this situation, my visits to this area were brief. Hence, Loudwater gets a rather hasty treatment here, and Hellgate Keep, Llorkh, Orlbar, and Zelbross are relegated to the section of this book entitled .Other Places of Note in the North..
Even before the shadow of evil fell across Hellgate Keep, these lands were perilous. Bordered on one side by wild mountains and on the other by a vast wild wood.perhaps the largest in all Faerūn.this is a territory roamed by monsters and rapacious humanoids.
The Dale also holds the ruins of Netheril, notably the Fallen Lands, across the mountains to the east of the Vale. The ruins are haunted by fearsome creatures warped by the fell magics of decadent human sorcerers.