The Spine of the World is a big frozen mountain range in the northern part of Faerûn.
Most people consider this the ultima thule, the utmost North - well, technically, the Icewind Dale and Frozenfar are the farthest places and they're just on the other side of the mountains, but at least there are passes leading there, around the mountains... Beyond the mountain range the is just lots and lots of ice and generally unpleasant climate, and are very few habitable locations beside the aforementioned places.
The mountains are almost inpassable. The few passes around the mountains are open for a short time only - and there are also a lot of arctic monsters, yeti and frost giants in particular, that tend to make passing through the mountains a real picnic. Not to even mention all typical mountain inhabitants, all the way from orcs and goblins to bigger nasties.
Under the mountains lie various dwarf halls, most of which have been abandoned long time ago and are now held by goblinoids, or in some cases, something far more sinister. The most famous dwarf hall at the time must be Mithral Hall in the southeastern end of the mountains, reclaimed by Bruenor Battlehammer in 1356 DR.
Sources: Forgotten Realms Campaign Setting
A Forgotten Realms novel by R.A. Salvatore, published in 1999. 2nd part of the Paths of Darkness series (and 12th in the whole series, unless I miscalculated severely).
The book was a pretty odd in that it was a dramatic change from the previous Drizzt books. First of all, the characters: The book ignores Drizzt and friends almost completely and focuses on Wulfgar the barbarian (recently returned from being tortured in the netherworld, quite shaken obviously) and - of all things - a peasant girl called Meralda whom the local noble falls in love with... which leads to the second odd thing, the plot. The book follows the life of Wulfgar as a tavern bouncer and a loud drunk in Luskan, the schemings and (mis)adventures of Wulfgar and his new "friend" Morik - and on the other side, Meralda's difficult and dramatic love story. In the end, these two tales then overlap. Quite an entertaining story as a whole and freshener of a looong series.